in music, arrangement or composition for instruments. Most authorities make little distinction between the words instrumentation and orchestration. Both deal with musical instruments (musical instrument) and their capabilities of producing various timbres (timbre) or colours. Orchestration (orchestra) is somewhat the narrower term since it is frequently used to describe the art of instrumentation as related to the symphony orchestra. Instrumentation, therefore, is the art of combining instruments in any sort of musical composition, including such diverse elements as the numerous combinations used in chamber (chamber music) groups, jazz bands, rock ensembles, ensembles employing chorus, symphonic bands, and, of course, the symphony orchestra. Included under this designation are the various instrumental groups that play non-Western music, such as the gamelan orchestras of Bali and Java and the traditional ensembles of India, Africa, the Far East, and the Middle East. (For treatment of the instruments themselves, see the articles musical instrument, percussion instrument, stringed instrument, keyboard instrument, wind instrument, and electronic instrument.)

In Western music there are many standard or traditional groups. Although there is great variability, depending on the composer and the era, a modern symphony orchestra often comprises the following instruments (wind instrument):

1. Woodwinds (woodwind): three flutes (flute), piccolo, three oboes (oboe), English horn (cor anglais), three clarinets (clarinet), bass clarinet, three bassoons (bassoon), contrabassoon (double bassoon).

2. Brass (brass instrument): four trumpets (trumpet), four or five French horns (French horn), three trombones (trombone), tuba.

3. Strings (stringed instrument): two harps (harp), first and second violins (violin), violas (viola), violoncellos (cello), double basses (double bass).

4. Percussion (percussion instrument): four timpani (played by one player), several other instruments (shared by a group of players).

The orchestra has arrived at this complement through centuries of evolution; the present size is needed to perform repertoire from the Baroque, Classical, Romantic, and Impressionistic periods, as well as the repertoire of the 20th century.

The various sections, with the exception of percussion, divide themselves in somewhat the same manner as a choir. The woodwinds, for example, divide into flutes (sopranos), oboes (altos), clarinets (tenors), and bassoons (basses), although this distinction must be greatly qualified. Instrumental range is larger than vocal range, and the clarinets of an orchestra may play higher than the flutes in a woodwind passage.

The standard instrumental groups of Western chamber music include the string quartet (two violins, viola, and violoncello), the woodwind quintet (flute, oboe, clarinet, French horn, and bassoon), the combinations employed in sonatas (one wind or stringed instrument with piano), and the brass quintet (frequently two trumpets, French horn, trombone, and tuba). In addition to these standard groups there are, however, hundreds of other possible combinations.

Other groups that deserve mention are those used in the popular music of the 20th century. The dance band, popular in the 1930s and 1940s, consisted of five saxophones, four trumpets, four trombones, double bass, piano, guitar, and drums. The basic rock ensemble consists of two electric guitars, electric bass, electronic organ (doubling electric piano), drums, and frequently one or more singers. The concert band, which is particularly popular in North America, consists of mixed wind and percussion players totalling from about 40 to well beyond 100 players.

The music of the non-Western world is most frequently performed by groups of chamber music size. In this category would fall the music played by the Javanese gamelan orchestra (consisting mainly of tuned gongs and other metal instruments), Japanese gagaku music (performed on flutes, mouth organs, lutes, drums, and gongs), and Chinese music (with a traceable history of about 4,000 years) consisting of sacred, folk, chamber, and operatic music.

The approach to the art of instrumentation is naturally greatly influenced by the type of group for which the composer is writing. He cannot treat a string quartet or a group of brass instruments in the same manner as he would a symphony orchestra. In general, the larger and more diverse the instrumental group, the more coloristic possibilities it presents to the composer. The smaller instrumental groups often have a sound character of their own, and the composer is challenged to find new and interesting ways to deal with this limitation.

The symphony orchestra has had definite traditions in relation to orchestration. The composer of the 18th century was likely to use the orchestral instruments at least part of the time in the following manner: the flutes doubling the same part as the first violins (frequently the melody); the oboes doubling the second violins or the first violins in octaves; the clarinets (by the end of the century) doubling the violas; and the bassoons doubling the violoncellos and double basses. French horns were often used as harmonic “filler” and in conjunction with every section of the orchestra because of their ability to blend easily with both stringed and wind instruments.

These traditional doublings were not so often used in the orchestration of the 19th and 20th centuries because of the great improvement in the making of wind instruments and their consequent ability to function in a solo capacity. Wind instruments became used more and more for colouring; the flutes, for instance, were noted for their bright tone quality and great technical agility, the clarinets for all the aforementioned qualities, and the bassoons for their special tone quality. Brass instruments had to await the development of valves (valve), which increased greatly the musical proficiency of brass players and overcame previous typecasting of these instruments as bugles and hunting horns.

The string quartet has long been considered one of the greatest challenges to the composer because the contrast to be achieved by changing from one type of instrument when writing for a full orchestra is simply not available. The composer has had to rely on varying timbres to be arrived at by different playing techniques, such as pizzicato (plucking the strings), tremolo (the quick reiteration of the same tone), sul ponticello (bowing (bow) near the bridge of the instrument), sul tasto (bowing on the fingerboard), the use of harmonics (dividing the string in such a way as to produce a high flutelike tone (overtone)), col legno (striking the strings with the wood of the bow), and many special bowing techniques.

Special playing techniques also can alter the timbres of wind instruments. For instance, on many, tremolos can be played on two different notes. Some wind instruments—and the flute is particularly agile in this respect—can produce harmonics. Flutter tonguing (produced by a rapid rolling movement of the tongue) is possible on most wind instruments; so are many other tonguing techniques that affect the quality of sound in orchestration.

The string mute is a device that softens the tone of the instrument. Muting is also used by brass instruments, particularly the trumpet and trombone, a development that took place in 20th-century popular music and then came into common use in all types of music. Mutes—of which there are various kinds—provide the trumpet and trombone with a different tone colour. Mutes on woodwind instruments have been experimented with, but the results have not been satisfactory.

Percussion instruments have become a favourite source of colour for the 20th-century composer, both in the concert and popular fields. Instruments from all over the world are now commonly available and are divided into two categories: of definite and of indefinite pitch. The former include the xylophone, marimba, vibraphone, glockenspiel, timpani, and chimes. Instruments of indefinite pitch exist by the hundreds. Some of the more common ones are the snare drum, tenor drum, tom-tom, bass drum, bongos, Latin American timbales, many types of cymbals, maracas, claves, triangles, gongs, and temple blocks.

The availability of these instruments and the great improvement in percussion playing has resulted in an enormous increase in the number of compositions for percussion instruments. The percussion ensemble, a group of from four to eight players, is a chamber group that has existed only in the 20th century, particularly since the late 1940s. One of the interesting features of such an ensemble is that each player in it is capable of playing many instruments. An ensemble of four players, for instance, can easily handle 25 or 30 instruments, once again showing the rich palette available in a single composition.

Since the 17th century, keyboard instruments have played an important role in orchestration. Those commonly available today are the harpsichord, celesta, organ (both pipe and electronic), and electric piano, in addition to the instrument for which most of the standard literature has been written—the piano. Keyboard instruments vary greatly in the manner in which they produce a sound: the harpsichord has quills that pluck the strings; the piano has hammers that strike the strings; the celesta has hammers that strike a metal bar; the pipe organ sends air through a pipe; the electronic organ employs electronic oscillators to produce its sound. The resulting colours are naturally very different.

The piano, with its wide range (more than seven octaves), has been used in conjunction with virtually every instrument and instrumental combination. In the 18th century it gradually replaced the harpsichord as the common keyboard instrument because of the piano's ability to alter dynamics rapidly and its ability to sustain sounds. There is a vast amount of literature for the piano as the accompanying instrument in sonatas, partly because the piano can function as a “one-man orchestra.” Many composers of the 20th century have discovered facets of the piano that had been previously ignored. The inside of the grand piano is a harplike body that has presented many new possibilities to the composer, such as the “prepared” piano. To prepare a piano, objects such as bolts, pennies, and erasers are inserted between the strings, thus producing many different sounds. The piano strings can be plucked or played with percussion mallets and can produce harmonics in the manner of non-keyboard stringed instruments, much to the dismay of piano tuners and traditional pianists.

The electric piano is one of a number of instruments that have gained in popularity in recent times. These instruments either produce sound by means of electronic oscillators or are amplified (amplifier) acoustic instruments. The sound produced by ensembles playing this type of instrument is distinctive. The rock ensemble is the best known, but rock musicians are by no means the only instrumentalists to employ electric instruments. For the composer, amplified or electric instruments pose certain problems. Balances can be achieved or ruined simply by turning an amplifier up or down. The timbres produced by rock ensembles and other groups employing electronics are unusual for a number of reasons. The electric guitar has such devices as reverberation controls, “wa-wa” pedals, and filters that enable the performer to change timbre radically in the middle of a performance. Composers since the early 1960s, being much concerned with coloristic possibilities of instruments, have found the electronic ones most attractive.

The largest quantity of literature in Western music has been written for the chorus. The choir, an instrument capable of great subtleties of colour, has been a favourite of composers for centuries. The range of most individual singing voices is rather limited. Choral singers (choral music), who usually have a limited amount of training, are capable of a range of about an octave and a fifth, which is considerably smaller than the range of individual instruments. Singers are usually not capable of singing wide leaps, that is to say, notes that are far apart in range. Great skill is required in the musical setting of the text in a choral work. Attention must be paid to the vocal qualities of vowel sounds as well as to the way in which the consonants are treated.

For centuries composers have been intrigued with the combination of voices and instruments, and many of the most important compositions in Western music have been written for chorus and orchestra. Almost every major composer of the past three centuries has written for choir and large instrumental ensembles.

The development of the art of using instruments for their individual properties did not really begin in Western music until about 1600. The known history of musical instruments, however, has been traced back 40,000 years, although nothing is known about the music these early instruments produced. The Greeks left mostly musical theories and only a small amount of extant music. The Romans used instruments particularly in military bands, but, again, little is known of their specific use. The music of the Middle Ages and Renaissance was primarily vocal, although instruments were frequently used in compositions to accompany (accompaniment) or reinforce the individual vocal line. Stringed, brass, woodwind, and percussion instruments were added not so much for their coloristic potential but because of their availability. Another practice in the Middle Ages was to make literal instrumental versions of vocal compositions, which, of course, has rather little in common with the modern art of instrumentation.

Orchestration in a modern sense probably began in the 16th century with Giovanni Gabrieli (Gabrieli, Giovanni), organist of St. Mark's in Venice. He was the first composer to sometimes designate specific instruments for each part in a composition, as in his Sacrae symphoniae (1597). Claudio Monteverdi (Monteverdi, Claudio) made important contributions to the art of orchestration. His opera Orfeo was first performed at Mantua (now Mantova, Italy) in 1607 with an orchestra of about 40 instruments, including flutes, cornetts, trumpets, trombones, strings, and keyboard instruments. For the first time, a composer, in order to heighten certain dramatic moments, specified exactly which instruments were to be used.

The century after the first performance of Orfeo was characterized by a rise in the use of stringed instruments that were similar to the modern ones. Although that trend helped set the stage for the modern orchestra, it was not a period that made great strides in the art of orchestration: the prevalent practice of writing out only the melody and the bass line of a composition did not lend itself easily to creative scoring. By the end of the 17th century, however, the groundwork had been laid for new developments. Instruments and instrumentalists had improved steadily. Johann Sebastian Bach (Bach, Johann Sebastian) created works that occasionally exploited the coloristic capabilities of instruments but in a rather limited way. In some of Bach's music the stringed instruments are played pizzicato, although this practice had already been employed by Monteverdi. Bach also wrote for muted strings. Wind instruments were treated occasionally for their special sounds, although more frequently they were simply employed on a musical line that their range happened to fit.

Handel (Handel, George Frideric), whose life covered the same period as Bach's, had a keener sense of orchestral effect. He introduced the clarinet into his orchestra, although it was not to become standard until the 19th century, and in his operas Handel often used instrumental colour in a way that did not become common practice until much later. Jean-Philippe Rameau (Rameau, Jean-Philippe), the leading French composer of the 18th century, also contributed much to the development of orchestration. Rameau, like Handel, was principally famous as an opera composer, and the overtures and dances of his operas represent the most advanced uses of instruments during that period. Rameau was probably the first composer (musical composition) to treat each instrument of the orchestra as a separate entity, and he introduced interesting and unexpected passages for flutes, oboes, and bassoons.

By the middle of the 18th century the symphony orchestra was beginning to resemble the modern instrumental group, yet it was still considerably smaller. The orchestra at the court of Mannheim (Mannheim school), Germany, consisted of 20 violins, four violas, four cellos, two double basses, two flutes, two oboes, two bassoons, four French horns, one trumpet, and kettledrums. Baroque composers frequently could not count on a fixed orchestra and therefore had to write the various parts so that they could be played on more than one instrument. The contrapuntal style (counterpoint) that prevailed from the time of Monteverdi until the mid-18th century usually meant simply assigning instruments to each line in a composition; the basic consideration was whether that line stayed within the range of the chosen instrument. The fixed personnel of such orchestras as the Mannheim group, therefore, freed the composers to experiment with the capabilities of the instruments within the group. Musical style was also changing, the contrapuntal style of the Baroque giving way to a style that relied more heavily on melodic invention supported by harmony.

One of the more important composers of the period between the Baroque and Classical eras was Johann Sebastian's son, Carl Philipp Emanuel Bach (Bach, Carl Philipp Emanuel). In C.P.E. Bach's symphonies (symphony) the strings become melodic instruments, and the winds—two flutes, two oboes, one or two bassoons, two horns—fill out chords and provide body to the orchestration.

The Classical era, which covers roughly the second half of the 18th century, is one of the most significant periods in the development of orchestration. The most talented composers of this period were Mozart (Mozart, Wolfgang Amadeus) and Haydn (Haydn, Joseph). Many important developments took place during this time. The orchestra became standardized. The Classical orchestra came to consist of strings (first and second violins, violas, violoncellos, and double basses), two flutes, two oboes, two clarinets, two bassoons, two or four French horns, two trumpets, and two timpani. Toward the end of his career, in the London Symphonies, Haydn introduced clarinets as part of the woodwind section, a change that was to be permanent. Haydn also introduced the following innovations: trumpets were used independently instead of always doubling the horns, cellos became separated from the double basses, and woodwind instruments were often given the main melodic line. In the Military Symphony (No. 100) Haydn introduced some percussion instruments not normally used in the orchestras of this time, namely, triangle, hand cymbals, and bass drum; and, what is still more unusual, they are employed in the second movement, which in the Classical tradition is normally the slow movement.

In Haydn's music (melody) a method of composition appeared that had a bearing on orchestration. This consisted of the conscious use of musical motives; motive is defined in the Harvard Dictionary of Music as: “The briefest intelligible and self-contained fragment of a musical theme or subject.” Perhaps the best known musical motive in Western music is the four-note group with which Beethoven's Fifth Symphony begins. These musical cells became the musical building blocks of the Classical period, particularly in the middle or development section of a movement, with the composer moving the musical motive from instrument to instrument and section to section, giving a new facet to the orchestration. The art of orchestration was thus becoming a major factor in the artistic quality of the music.

Mozart, too, was responsible for great strides in the creative use of instruments. His last two symphonies (Nos. 40, K 550, and 41, K 551) are among the most beautifully orchestrated works of this or any period. For his 17 piano concertos, Mozart exhaustively explored the combination of piano and orchestra.

Beethoven began his career under the influence of the Classical composers, particularly Haydn, but during his lifetime he transformed this heritage into the foundation of a new musical practice that was to become known as Romanticism. The Classical composers for the most part attempted to orchestrate with a sense of grace and beauty. Beethoven occasionally made deliberate use of new, intense, often even harsh orchestral sounds. He also, in his later symphonies, augmented the orchestra with a piccolo, contrabassoon, and third and fourth horn. The Ninth Symphony has one passage calling for triangle, cymbals, and bass drum, a combination identified with the imitations of Turkish Janissary music in vogue in previous years.

The Romantic era was characterized by great strides in the art of instrumentation, and, in fact, the use of instrumental colour became one of the most salient features of this music. The piano really came into its own as a source of interesting sonorities; the orchestra expanded in size and scope; new instruments were added; and old instruments were improved and made more versatile. The Romantic period saw the appearance of the first textbook on the subject of orchestration. It was the French composer Hector Berlioz (Berlioz, Hector)' Traité d'instrumentation et d'orchestration modernes (1844; Treatise on Instrumentation and Orchestration, 1856). Berlioz was one of the most individual orchestrators in the history of music, and his Symphonie fantastique (1830) is one of the most remarkable pieces of music to come out of this era. Berlioz made use of colour to depict or suggest events in his music, which was frequently programmatic in character. He called on large forces to express his musical ideas, an idea that persisted throughout the 19th century and into the 20th. Berlioz' Grande Messe des morts (Requiem, 1837) calls for four flutes, two oboes, two English horns, four clarinets, 12 French horns, eight bassoons, 25 first violins, 25 second violins, 20 violas, 20 violoncellos, 18 double basses, eight pairs of timpani, four tam-tams (a type of gong), bass drum, and 10 pairs of cymbals; four brass choirs placed in various parts of the hall, each consisting of four trumpets, four trombones, two tubas, and four ophicleides (a large, now obsolete brass instrument); and a chorus of 80 sopranos, 80 altos, 60 tenors, and 70 basses.

The coloristic ideas in Berlioz' music were carried on in various ways by other important 19th-century composers and reached a culmination in the music of the German composer Richard Strauss (Strauss, Richard) and the Austrian Gustav Mahler (Mahler, Gustav)—both of whom demanded a virtuoso orchestra—and were orchestrated in a complex fashion, although Mahler was capable of very delicate effects.

Claude Debussy (Debussy, Claude) in France was probably the most important composer of the Impressionistic era (Impressionism), which lasted roughly from 1880 until the turn of the century. The Impressionist composers attempted to describe scenes and evoke moods by the use of rich harmonies and a wide palette of timbre. No composer ever handled the colours of the orchestra with greater subtlety. Naturally, this is also dependent on his use of harmony, melody, and rhythm, but the dominant impression of a Debussy work is focussed on his use of orchestral instruments to create light and shadows. Works that exemplify his techniques are Prélude à l'après-midi d'un faune (Prelude to the Afternoon of a Faun; 1894), Nocturnes (1899), and La Mer (The Sea; 1905). In Nocturnes he uses a wordless women's chorus as a section of the orchestra, functioning as another source of timbre rather than as the transmitter of a text.

Many of the composers who followed Debussy and Mahler brought about radical changes in the use of the orchestra. A good example of some of these changes is in The Rite of Spring (1913), by the Russian-born composer Igor Stravinsky (Stravinsky, Igor). The strings frequently do not assume a dominant role but, rather, often play music that is subservient to the brass or woodwinds. Percussion instruments greatly increased in importance and have continued to do so. In 1931, Edgard Varèse (Varèse, Edgard) composed an important work, Ionisation, for 13 percussion players, a landmark in the emergence of percussion instruments as equal partners in music.

The period between World War I and World War II was dominated by two main schools of composers with vastly differing results for orchestration. One was responsible for the Neoclassical style; the other, gathered around the Austrian composer Arnold Schoenberg (Schoenberg, Arnold), drew heavily on the Romantic movement for its direction. The Neoclassical composers sought to free music from the influence of Impressionism. Whereas the Impressionist and Romantic composers had frequently employed the instrumental forces at hand to create a deliberate sense of vagueness, the Neoclassical composers, beginning in about 1917 with a group in France known as Les Six (Six, Les), attempted to recreate the clarity of the Classical period by turning to models found in the popular music of the period, the music of the dance halls and cabarets. The Neoclassical composers also turned away somewhat from the orchestra as a medium, finding the forces of chamber music more suitable for their ideals. Neoclassical music returned to a clearer concept of “sections” in orchestration. The music of a composer such as Paul Hindemith in Germany is closer to the music of Mozart in its sense of instrumentation than it is to Romanticism or Impressionism.

The music of Schoenberg and his fellow Austrian Alban Berg drew heavily on the Romantic movement and eventually became known as Expressionism, which stressed inner experience. Emphasis on the inner man produced a music that was thick, dark, and intense.

In the first half of the 20th century electronic music emerged, although it did not become important until after 1950. The principal reasons for the inclusion here of electronic music are that electronic sounds, either taped or live, frequently are included in a composition combined with traditional instruments, and it has had a decided influence on orchestration. (For a treatment of historical and compositional aspects of electronic music, see electronic music.) By the 1960s many composers were writing works for electronic sounds and instruments. The electronic sounds provide a dimension to instrumentation never before possible. A number of things are noteworthy. Electronic sounds are capable of incredibly subtle changes of timbre, pitch, and mode of attack. When combined with traditional instruments they add a rich new spectrum of colour. This in turn has influenced the composer to attempt to produce “electronic” sounds with standard instruments. The result has been a great extension of the sound possibilities of Western instruments.

Another 20th-century trend was away from large orchestras and toward chamber ensembles, often of nontraditional combinations. Compositions for such ensembles may excel in economy of means and explore individual instrumental timbres. To achieve this, unusual playing techniques may be required.

Much of music outside the West has entirely different aesthetic aims; the music of the Hindu world, best known to the West through the classical music of India, provides an example. Indian music always has had strong ties with mythology and religion and thus produced an art that is as different from Western music as Hinduism is from Christianity. It achieves unity through similarity rather than through change and is based on a more purely sensual approach. Hindu music is divided, for example, into ragas (raga), or melody types. The word raga means colour or mood. Combined with the ragas are tālas (tala), or rhythmic patterns. The possible combinations of tālas and ragas are many, producing a music that is wonderfully subtle.

The instruments for this music consist of various drums made of terra-cotta, wood, or metal; cymbals also serve as percussion instruments. Probably the instrument best known to Western audiences is the tabla, a two-drum set capable of very subtle changes in sound. The two best known stringed instruments are the sitar (plucked) and the tamboura, a four-stringed instrument that provides the omnipresent drone accompaniment. In addition, there are various wind instruments, such as the bamboo flute and the sheh'nai (oboe).

Balinese (Bali) and Javanese (Java) music is centred on the gamelan orchestra, the instruments of which include the saron and gender metallophones (like xylophones but with metal, not wooden, keys), the gambang kayu xylophone, tuned gongs, flutes, and the rebab, a violin-like instrument with two strings. All the instruments follow the same nuclear melody but elaborate it in different ways. The heavy reliance on tuned percussion instruments has given this music a brilliant quality that Western audiences have found extremely attractive. The gamelan orchestra, for instance, influenced Debussy, who first heard the music at the Paris Exposition in 1889.

The approach to instrumentation in the music of India and Bali is quite different from that of Western music. The concept of contrast created through the various “choirs” of the Western orchestra is not a primary concern. In Indian music a sameness of colour is created through the use of the drone played on the tamboura. This is not to say that this music is uncolourful but that a specific timbre is established for an entire composition (musical composition). Since the time of Debussy, Western composers have come increasingly into contact with, in particular, the music of India, Bali, and Japan. A comparison of Balinese gamelan music with the Sonatas and Interludes for prepared piano by the 20th-century American composer John Cage shows how profound this influence can be.

A practice that has been much employed in the 20th century, although by no means confined to it, has been the writing of arrangements and transcriptions. Though little distinction is made between the two, there are, at least in current practice, differences. A transcription is essentially the adaptation of a composition for an instrument or instruments other than those for which it was originally written. An arrangement is a similar procedure, although the arranger often feels free to take musical liberties with elements of the original score. This is especially true of arrangements for jazz or rock groups and arrangements of popular compositions or songs from musical comedies.

In the 18th and 19th centuries, chamber (chamber music) and orchestral (orchestra) music was transcribed, or “arranged,” for the piano for the purpose of study and, of course, for the pleasure of playing at home the music that had been heard at a concert. This practice has continued. Piano versions of many 18th- and 19th-century orchestral works exist in two- and four-hand arrangements. Another common practice is to reduce the orchestral parts of concertos to a keyboard version to enable students to study and play these works without an orchestra.

The symphonic band, despite its popularity in Great Britain and North America, was faced with a dearth of repertoire written specifically for it. In the past, one answer was to transcribe orchestral works for band, substituting particularly the clarinets, with their wide pitch range, for the strings of the symphony orchestra. The necessity for that substitution is no longer so great because in recent times composers have written much more music specifically for the symphonic band.

The dance band predominant in the 1930s and 1940s is treated roughly in the following way by arrangers: the saxophones carry the melody more frequently than the other sections; the trumpets provide embellishment or figures that work around the melody; the trombones either are combined with the trumpets or serve as a melodic instrument; the piano and guitar provide harmonic filler; and the double bass and drums set the rhythm.

The jazz or rock arranger has done much more than simply transcribe the keyboard version of a song. All forms of popular music in the 20th century have been involved in the art of improvising (improvisation). Musicians working in this field almost always embellish the music as they perform it. The jazz or rock arranger in a sense improvises on manuscript paper. In making an arrangement for a group of musicians the arranger will embellish both the harmonic structure and the melody of the composition; or the arrangement will be worked out in rehearsal and memorized or written down later. Usually, the arranger keeps enough of the original material to enable the listener to recognize the source. His skill depends on how well he can manipulate the materials of the original and on his originality in scoring the composition for the group at his disposal. The men and women who work in this field are frequently composers of popular music themselves.

in technology, the development and use of precise measuring equipment. Although the sensory organs of the human body can be extremely sensitive and responsive, modern science and technology rely on the development of much more precise measuring and analytical tools for studying, monitoring, or controlling all kinds of phenomena.

Some of the earliest instruments of measurement were used in astronomy and navigation. The armillary sphere, the oldest known astronomical instrument, consisted essentially of a skeletal celestial globe whose rings represent the great circles of the heavens. The armillary sphere was known in ancient China; the ancient Greeks were also familiar with it and modified it to produce the astrolabe, which could tell the time or length of day or night as well as measure solar and lunar altitudes. The compass, the earliest instrument for direction finding that did not make reference to the stars, was a striking advance in instrumentation made about the 11th century. The telescope, the primary astronomical instrument, was invented about 1608 by the Dutch optician Hans Lippershey (Lippershey, Hans) and first used extensively by Galileo.

Instrumentation involves both measurement and control functions. An early instrumental control system was the thermostatic furnace developed by the Dutch inventor Cornelius Drebbel (Drebbel, Cornelis) (1572–1634), in which a thermometer controlled the temperature of a furnace by a system of rods and levers. Devices to measure and regulate steam pressure inside a boiler appeared at about the same time. In 1788 the Scotsman James Watt invented a centrifugal governor to maintain the speed of a steam engine at a predetermined rate.

Instrumentation developed at a rapid pace in the Industrial Revolution of the 18th and 19th centuries, particularly in the areas of dimensional measurement, electrical measurement, and physical analysis. Manufacturing processes of the time required instruments capable of achieving new standards of linear precision, met in part by the screw micrometer, special models of which could attain a precision of 0.000025 mm (0.000001 inch). The industrial application of electricity required instruments to measure current, voltage, and resistance. Analytical methods, using such instruments as the microscope and the spectroscope, became increasingly important; the latter instrument, which analyzes by wave length the light radiation given off by incandescent substances, began to be used to identify the composition of chemical substances and stars.

In the 20th century the growth of modern industry, the introduction of computerization (computer), and the advent of space exploration spurred still greater development of instrumentation, particularly of electronic devices. Often a transducer, an instrument that changes energy from one form into another (such as the photocell, thermocouple, or microphone) is used to transform a sample of the energy to be measured into electrical impulses that are more easily processed and stored. The introduction of the electronic computer in the 1950s, with its great capacity for information processing and storage, virtually revolutionized methods of instrumentation, for it allowed the simultaneous comparison and analysis of large amounts of information. At much the same time, feedback systems were perfected in which data from instruments monitoring stages of a process are instantaneously evaluated and used to adjust parameters affecting the process. Feedback systems are crucial to the operation of automated processes.

Most manufacturing processes rely on instrumentation for monitoring chemical, physical, and environmental properties, as well as the performance of production lines. Instruments to monitor chemical properties include the refractometer, infrared analyzers, chromatographs, and pH sensors. A refractometer measures the bending of a beam of light as it passes from one material to another; such instruments are used, for instance, to determine the composition of sugar solutions or the concentration of tomato paste in ketchup. Infrared analyzers can identify substances by the wavelength and amount of infrared radiation that they emit or reflect. Chromatography, a sensitive and swift method of chemical analysis used on extremely tiny samples of a substance, relies on the different rates at which a material will adsorb different types of molecules. The acidity or alkalinity of a solution can be measured by pH sensors.

Instruments are also used to measure physical properties of a substance, such as its turbidity, or amount of particulate matter in a solution. Water purification and petroleum-refining processes are monitored by a turbidimeter, which measures how much light of one particular wavelength is absorbed by a solution. The density of a liquid substance is determined by a hydrometer, which measures the buoyancy of an object of known volume immersed in the fluid to be measured. The flow rate of a substance is measured by a turbine flowmeter, in which the revolutions of a freely spinning turbine immersed in a fluid are measured, while the viscosity of a fluid is measured by a number of techniques, including how much it dampens the oscillations of a steel blade.

Instruments used in medicine and biomedical research are just as varied as those in industry. Relatively simple medical instruments measure temperature, blood pressure (sphygmomanometer), or lung capacity (spirometer). More complex instruments include the familiar X-ray machines and electroencephalographs and electrocardiographs, which detect electrical signals generated by the brain and heart, respectively. Two of the most complex medical instruments now in use are the CAT (computerized axial tomography) and NMR (nuclear magnetic resonance) scanners, which can visualize body parts in three dimensions. The analysis of tissue samples using highly sophisticated methods of chemical analysis is also important in biomedical research.