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Choir
A body of singers who perform together as a group 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. Choirs may sing without instrumental accompaniment, with the accompaniment of a piano or pipe organ, with a small ensemble (e.g., harpsichord, cello and double bass for a Baroque piece), or with a full orchestra of 70 to 100 musicians.

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.

The conductor or choral director typically stands on a raised platform and he or she may or may not use a baton; using a baton gives the conductor's gestures greater visibility, but many choral conductors prefer conducting with their hands for greater expressiveness, particularly when working with a smaller ensemble. In the 2010s, most conductors do not play an instrument when conducting, although in earlier periods of classical music history, leading an ensemble while playing an instrument was common. In Baroque music from the 1600s to the 1750s, conductors performing in the 2010s may lead an ensemble while playing a harpsichord or the violin (see Concertmaster). Conducting while playing a piano may also be done with musical theatre pit orchestras. Communication is typically non-verbal during a performance (this is strictly the case in art music, but in jazz big bands or large pop ensembles, there may be occasional spoken instructions). However, in rehearsals, the conductor will often give verbal instructions to the ensemble, since they generally also serve as an artistic director who crafts the ensemble's interpretation of the music.

In addition to leading of singing in which the congregation participates, such as hymns and service music, some church choirs sing full liturgies, including propers (introit, gradual, communion antiphons appropriate for the different times of the liturgical year). Chief among these are the Anglican and Roman Catholic churches; far more common however is the performance of anthems or motets at designated times in the service.

Nationally, male students are enrolled in choir at much lower numbers than their female students. The music education field has had a longtime interest in the "missing males" in music programs. Speculation as to why there aren't as many boys in choir, and possible solutions vary widely. One researcher found that boys who enjoy choir in middle school may not always go on to high school choir because it simply doesn't fit into their schedules. Some research speculates that one reason that boys' participation in choir is so low is because the U.S. does not encourage male singers. Often, schools will have a women's choir, which helps the balance issues mixed choirs face by taking on extra female singers. However, without a men's choir also, this could be making the problem worse by not giving boys as many opportunities to sing as girls. Other researchers have noted that having an ensemble or even a workshop dedicated to male singers can help with their confidence and singing abilities.

The earliest notated music of western Europe is Gregorian chant, along with a few other types of chant which were later subsumed (or sometimes suppressed) by the Catholic Church. This tradition of unison choir singing lasted from sometime between the times of St. Ambrose (4th century) and Gregory the Great (6th century) up to the present. During the later Middle Ages, a new type of singing involving multiple melodic parts, called organum, became predominant for certain functions, but initially this polyphony was only sung by soloists. Further developments of this technique included clausulae, conductus and the motet (most notably the isorhythmic motet), which, unlike the Renaissance motet, describes a composition with different texts sung simultaneously in different voices. The first evidence of polyphony with more than one singer per part comes in the Old Hall Manuscript (1420, though containing music from the late 14th century), in which there are apparent divisi, one part dividing into two simultaneously sounding notes.

The interaction of sung voices in Renaissance polyphony influenced Western music for centuries. Composers are routinely trained in the "Palestrina style" to this day, especially as codified by the 18c music theorist Johann Joseph Fux. Composers of the early 20th century also wrote in Renaissance-inspired styles. Herbert Howells wrote a Mass in the Dorian mode entirely in strict Renaissance style, and Ralph Vaughan Williams's Mass in G minor is an extension of this style. Anton Webern wrote his dissertation on the Choralis Constantinus of Heinrich Isaac and the contrapuntal techniques of his serial music may be informed by this study.