A choir, chorale, or chorus is a musical ensemble of singers. Choral music, in turn, is the music written specifically for such an ensemble to perform.
A body of singers who perform together is called a choir or chorus, The former term is very often applied to groups affiliated with a church (whether or not they actually occupy the choir) and the second to groups that perform in theatres or concert halls, but this distinction is far from rigid. "Choir" has the secondary definition of a subset of an ensemble; thus one speaks of the "woodwind choir" of an orchestra, or different "choirs" of voices and/or instruments in a polychoral composition. In typical 18th to 20th century oratorios and masses, chorus or choir is usually understood to imply more than one singer per part, in contrast to the quartet of soloists also featured in these works.

Structure of choirs

Choirs are often led by a conductor or choirmaster. Most often choirs consist of four sections intended to sing in four part harmony, but there is no limit to the number of possible parts as long as there is a singer available to sing the part: Thomas Tallis wrote a 40-part motet entitled Spem in alium, for eight choirs of five parts each; Krzysztof Penderecki's Stabat Mater is for three choirs of 16 voices each, a total of 48 parts. Other than four, the most common number of parts are three, five, six and eight.
Choirs can sing with or without instrumental accompaniment. Singing without accompaniment is called a cappella singing (although the American Choral Directors Association[1] discourages this usage in favor of "unaccompanied", since a cappella denotes singing "as in the chapel" and much unaccompanied music today is secular). Accompanying instruments can consist of practically any instruments, from one to a full orchestra; for rehearsals a piano or organ accompaniment is often used even if a different instrumentation is planned for performance, or for rehearsing a cappella music. While Eastern Orthodox churches and some synagogues ban the use of instruments, in churches of the Western Rite the accompanying instrument is almost always an organ, although in colonial America, the Moravian Church used a string quartet. Many churches which use a contemporary worship format will have a band in the sanctuary to accompany the singing.

Beside the leading of singing in which the congregation participates such as hymns and service music, choirs still sing the full propers (introit, gradual, communion antiphons appropriate for the different times of the liturgical year) at a few churches, chiefly those of the Anglican or Roman Catholic churches; far more common however is the performance of an anthem at the offertory. Roman Catholic Churches use, at their discretion, additional orchestral accompaniment.

Choirs can be categorized by the voices:
Mixed choirs (i.e., with male and female voices). This is perhaps the most common type, usually consisting of soprano, alto, tenor and bass voices, often abbreviated as SATB. Often one or more voices is divided into two, e.g., SSAATTBB, where each voice is divided into two parts, and SATBSATB, where the choir is divided into two semi-independent four-part choirs. Occasionally baritone voice is also used (e.g., SATBarB), often sung by the higher basses. In smaller choirs with fewer men, SAB, or Soprano, Alto, and Baritone arrangements allow the few men to share the role of both the tenor and bass in a single part.
Male choirs, with the same SATB voicing as mixed choirs, but with boys singing the upper part (often called treble or boy soprano) and men singing alto (in falsetto), also known as countertenor. This format is typical of the British cathedral choir.
Female choirs, usually consisting of soprano and alto voices, two parts in each, often abbreviated as SSAA, or as soprano, soprano II, and alto, abbreviated SSA
Men's choirs, usually consisting of two tenors, baritone, and bass, often abbreviated as TTBB (or ATBB if the upper part sings falsetto in alto range like barbershop music, even though this notation is not normally used in barbershop music). Occasionally, a men's choir will have Basso Profondo, the lowest of all male vocal ranges.
Children's choirs, often two-part SA or three-part SSA, sometimes more voices. This includes boys' choirs.

Choirs are also categorized by the institutions in which they operate:
Church choirs
Collegiate choirs
School choirs
Community choirs (of children or adults)
Professional choirs, either independent (e.g.Philippine Madrigal Singers, Anúna) or state-supported (e.g., BBC Singers, National Chamber Choir of Ireland, Canadian Chamber Choir, Swedish Radio Choir etc.)

Finally, some choirs are categorized by the type of music they perform, such as
Symphonic choirs
Vocal jazz choirs
Show choirs, in which the members sing and dance, often in performances somewhat like musicals.

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