hollow vessel usually of metal, but sometimes of horn, wood, glass, or clay, struck near the rim by an interior clapper or exterior hammer or mallet to produce a ringing sound. Bells may be categorized as idiophones, instruments sounding by the vibration of resonant solid material, and more broadly as percussion instruments. The shape of bells depends on cultural environment, intended use, and material of construction. The walls vary from straight to convex, concave, hemispherical, barrel shaped (as in East Asia), and tulip shaped with sound bow (the bulge near the rim), as are all tower bells in the West. In cross section they may be round, square, rectangular, elliptical, or many-sided. Chinese bells often have lotus-shaped rims.

The strongest sound-producing vibrations of bells occur near the rim (in Western bells, in the sound bow), in contrast to hollow gongs, the vibrations of which are strongest in the centre. The acoustical structure of bell sound is complex and has been completely understood only in modern times. All bells contain an array of partials, or sound-wave frequencies of various pitches, but the tone of a musical bell consists of both harmonious partials and higher inharmonious partials. Western bells are invariably rung by a metal striker; Asian bells, except for metal-clappered hand and wind bells, are normally struck by a wooden hand mallet or swinging horizontal beam that engages the exterior wall. Asian bells are also devoid of the sound bow and never swing.

Bells are widely distributed geographically and usually possess a clearly defined cultural status. Legends surround them, and beliefs abound concerning their special powers—to induce rain or to dissolve storm clouds; to thwart demons when worn as amulets or when placed on animals, buildings, or conveyances; or to invoke curses and lift spells. The concept of their purifying action is ancient, as is their use in ritual, especially in the religions of eastern and southern Asia. The Chinese rang bells to communicate directly with spirits, and in Russian Orthodoxy, bells directly addressed the deity—hence, huge ones were cast by both peoples to lend greater authority. In both Buddhism and Christianity, bells are consecrated before being used liturgically, and in East Asia the fading tone of the bell is considered spiritually significant. In Roman Catholicism, bells have symbolized paradise and the voice of God.

Among the most basic and widespread uses of bells is signalling—marking significant points of ritual, calling to worship, tolling the hours, announcing events, rejoicing, warning, and mourning. In Christian and Buddhist monasteries, bells regulate daily routine, and medieval and Christian bells were named according to purpose: squilla for the refectory, nola for the choir, and so forth.

Bells have also been treasured as patriotic symbols and war trophies, and invaders quickly silenced those of the conquered in order to eliminate the most vivid symbol of resistance. Most cultures have made bells into artistic objects, with respect to shape, material, and ornamentation, and both Eastern and Western religions have incorporated symbolic motifs in the ornamentation of bells.

The ancient Chinese were the first to employ sequences of bells musically; such sequences are termed chimes, or bianzhong. In the West since the 9th century, small sets of bells (chimes) in stationary suspension and generally tuned diatonically (to the seven-note scale) have been common (see bell chime). Sets of tuned bells numbering at least 23 are termed carillons (carillon). Groups of two or more free-swinging bells peal; a single stationary bell in slow repetition tolls. All today may be operated electrically. Change ringing is a British form of pealing whereby 5 to 12 bells are rung in mathematical permutations. The zvon (“chime”) of the Russian Orthodox Church sounds repetitious rhythmic patterns. Sets of handbells ranging up to five octaves have been popular in England and the U.S. since the 19th century as a group method for producing melodies and simple harmonies. In the main the liturgical and utilitarian functions of bells have greatly diminished, whereas their musical usage has increased. A characteristic rhythmic unit in ensembles across central Africa is the double bell—two open bells made of iron that are joined so they can be held in the left hand and struck with a stick held in the right.

Forged and riveted metal bells antedate those of cast metal. The earliest bell founding (i.e., the casting of bells from molten metal) is associated with the Bronze Age. The ancient Chinese were superb founders, their craft reaching an apex during the Zhou dynasty (c. 1046–256 BC). Characteristic were elliptical temple bells with exquisite symbolic decorations cast onto their surfaces by the cire perdue, or lost wax, process.

European bell making was originally a monastic craft. The earliest Christian bells were of iron plates hammered square and riveted (resembling cowbells). Although bronze casting was practiced in pre-Christian Europe, it was not resumed to any extent until the 8th century.

In bell founding, molten metal (usually bronze) is poured into a mold consisting of an inner core and outer mold or cope contoured to a bell's profile. Most molds are faced with loam, those for handbells with sand. The liquid metal, heated to about 1,100° C (2,000° F) enters a hole at the top while being tamped (driven by a series of light blows) down through another. To avoid undesirable porosity, gases formed are allowed to escape. Cooling is carefully controlled to prevent the outer surface from cooling faster than the inner, thereby setting up a tension leading to later cracking. Large bells require a week or two to cool. When the mold is removed the rough casting of the bell is sandblasted and polished. If a certain pitch is required, small amounts of metal are ground from the bell's inner wall as it revolves. Bell metal, or bronze, is an alloy of copper and tin. Tin content may range from 13 percent in weight to 25 percent, rarely more. Tin increases brittleness, and large bells contain less than small ones do. Most carillon bells contain 20 percent.

Casting produced better toned bells by permitting greater wall thickness and more precise control of contour (now round). For centuries bells had a convex wall of uniform thickness, a shape termed a beehive or primitive bell. The wall was elongated for use in bell towers, and the rim was reinforced for more resonance and strength. Pitch was successfully controlled by the 9th century, when tuned sets of small bells (called cymbala) appeared.

By the 11th century, secular bell founders—often itinerant—were active, becoming dominant by the Renaissance. The lofty towers of Gothic architecture led to much larger, more resonant bells and gave rise to an archaic version of the present campaniform bell: tulip-shaped with a narrow, rounded top; a long, straight waist spreading outward at the bottom; and a flared mouth, or sound bow. By the 13th century this shape predominated. Until the 15th century, when a shape similar to the modern Western one emerged, it was slowly transformed, the waist becoming proportionately shorter and concave, the top broader, the shoulder squared, and the sound bow thickened.

Bell founding attained considerable prestige, and the introduction of gunpowder in the 14th century added cannon making to the founder's output. The founders of Belgium and the Netherlands surpassed all others, their stature growing as the carillon spread in that area in the 15th–18th century, their craft culminating with the 17th-century Dutch founders François and Pierre Hémony. The craft declined in the 19th century, particularly in the ability to tune accurately, but regained its excellence by the 20th.

Russian bell founding dates from the 13th century, and by the 16th, bells weighing many tons were made. The world's largest bell, the Tsar Kolokol III (Tsar Bell III) in Moscow, was cast in 1733–35, weighing about 400,000 pounds (180,000 kilograms); broken by fire in 1737, it never rang. English founders traditionally paid little attention to their bells' inner tuning of the partials, because their bell usages—change ringing and chimes—did not involve harmony. In the 20th century they adopted the partial tuning used in Belgium and The Netherlands.

The pellet bell, or crotal (a term also having many other meanings), a spherical vessel with loose pellets, has been historically regarded as a type of bell, but modern authorities now classify it as a rattle; jingle and sleigh bells are familiar examples. Of great antiquity, it shares many of the ritual and magical functions of bells.