in measurement, fundamental unit of length in the metric system and in the International Systems of Units (International System of Units) (SI). It is equal to approximately 39.37 inches in the British Imperial (British Imperial System) and United States Customary systems. The metre was historically defined by the French Academy of Sciences (Sciences, Academy of) in 1791 as ¹/_10,000,000 of the quadrant of the Earth's circumference running from the North Pole through Paris to the equator. The International Bureau of Weights and Measures in 1889 established the international prototype metre as the distance between two lines on a standard bar of 90 percent platinum and 10 percent iridium. By 1960 advances in the techniques of measuring light waves had made it possible to establish an accurate and easily reproducible standard independent of any physical artifact. In 1960 the metre was thus defined in the SI system as equal to 1,650,763.73 wavelengths of the orange-red line in the spectrum of the krypton-86 atom in a vacuum.

By the 1980s, advances in laser measurement techniques had yielded values for the speed of light in a vacuum of an unprecedented accuracy, and it was decided in 1983 by the General Conference on Weights and Measures that the accepted value for this constant would be exactly 299,792,458 metres per second. The metre is now thus defined as the distance traveled by light in a vacuum in ¹/_299,792,458 of a second.

in music, rhythmic pattern constituted by the grouping of basic temporal units, called beats (beat), into regular measures, or bars; in Western notation, each measure is set off from those adjoining it by bar lines. A time (or metre) signature, found at the beginning of a piece of music, indicates the number of beats in a measure and the value of the basic beat. For example, ³/₄ metre has three quarter-note beats per measure. The time signature implies that an accent regularly occurs on the first beat of each measure. Simple metres are duple (e.g., ²/₂, ²/₄), triple (³/₄, ³/₈), or quadruple (⁴/₄, ⁴/₈). Compound metres are also duple (⁶/₈, ⁶/₁₆), triple (⁹/₈), or quadruple (¹²/₈) but have time signatures that indicate the number of beats to be a multiple of three. Thus, in ⁶/₈, for example, both beats of the basic duple division are divisible into three subunits, yielding a total of six. Some metres that occur less frequently are neither duple nor triple (⁵/₄, ⁷/₄) but may be considered a combination of duple and triple—such as ²/₄+³/₄ or ³/₄+²/₄+²/₄.

The concept of regular rhythmic groups is traceable to the ancient beginnings of dance and poetry, which music primarily served to undergird. Specifications of metre first appeared in written music as early as about 1200, when short rhythmic formulas called rhythmic modes (rhythmic mode) (see rhythmic mode) came into use, implying repetition of simple triple patterns. From 1300 to 1600 both duple and triple metres were recognized in music theory, but, in practice, rhythm was often complex and involved combinations of metres. From the 17th to the 20th century, regular metres as used today became the standard. In the course of the 18th century, metre changes from movement to movement gained aesthetic significance equal to that of key and tempo distinctions in multimovement works. Such 20th-century composers as Igor Stravinsky and Béla Bartók accorded their extensive metrical manipulations a structural prominence previously reserved for melody and harmony.

in poetry, the rhythmic pattern of a poetic line. Various principles, based on the natural rhythms of language, have been devised to organize poetic lines into rhythmic units. These have produced distinct kinds of versification, among which the most common are quantitative, syllabic, accentual, and accentual-syllabic.

1. quantitative verse, the metre of classical Greek and Latin poetry, measures quantity, or the length of time required to pronounce syllables regardless of their stress. Various combinations of long and short syllables (the long syllables being roughly equivalent to twice the duration of the short syllables) constitute the basic rhythmic units. Quantitative verse has been adapted to modern languages but with limited success.

2. syllabic verse, most common in languages that are not strongly accented, such as the Romance languages and Japanese. It is based on a fixed number of syllables within a line, although the number of accents or stresses may be varied. Thus, the classic metre of French poetry is the alexandrine, a line of 12 syllables with a medial caesura (a pause occurring after the 6th syllable). The Japanese haiku is a poem of 17 syllables, composed in lines of 5/7/5 syllables each.

3. accentual verse, occurring in strongly stressed languages such as the Germanic. It counts only the number of stresses or accented syllables within a line and allows a variable number of unaccented syllables. Old Norse and Old English poetry is based on lines having a fixed number of strongly stressed syllables reinforced by alliteration. Accentual metres are evident in much popular English verse and in nursery rhymes; i.e., “One,́ two,́ Bucḱ |le˘ my˘ shoé .” In the late 19th century, the English poet Gerard Manley Hopkins used it as the basis for his poetic innovation “ sprung rhythm” (q.v.).

4. accentual-syllabic verse, the usual form of English poetry. It combines Romance syllable counting and Germanic stress counting to produce lines of fixed numbers of alternating stressed and unstressed syllables. Thus, the most common English metre, iambic pentameter, is a line of ten syllables or five iambic feet. Each iambic foot is composed of an unstressed syllable followed by a stressed syllable.

Variations within any of these regular metres are not only permissible but inevitable and de sirable. The words a˘|gaiń and for˘|lorn,́ for instance, may each constitute an iambic foot, but they are vastly different in quality. Even in the most formal metrical designs, the quality, pitch, and force of certain sounds, along with the interplay of other poetic devices such as assonance, consonance, alliteration, or rhyme may act to reinforce or obscure the basic metrical pattern.

The function of regular metre in poetry is complex. In its most primitive aspects, as in nursery rhymes or folk ballads, it creates the physical pleasure that any simple rhythmic acts such as rocking, swaying, trotting, or foot tapping provide. Used mimetically, it may be lulling, galloping, staccato, heavy and slow, or quick and light to match the content and emotional tone of the poem. In more sophisticated poetry, regular metre is a subtle and flexible device, organically integrated into the total poem through its sensitive interaction with the natural rhythms of speech and the meaning of words. Although the late 19th century and early 20th century witnessed a widespread rebellion against the restrictions of metrically regular poetry, the challenge of condensing an imaginative impulse into a formal framework still appeals to poets. See also foot; scansion.