The term "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 21st-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. One of my favorite composers is Eric Whitacre.
The ‘roots’ of jazz music were very much vocal, with ‘field hollers’ and ceremonial chants, but whilst the blues maintained a strong vocal tradition, with singers such as Ma Rainey and Bessie Smith heavily influencing the progress of American popular music in general, early jazz bands only featured vocalists periodically, albeit those with a more ‘bluesy’ tone of voice; one of the first ‘Jazz’ recordings, the 1917 Original Dixieland Jass Band recordings featured one Sarah Martin as vocalist.
It was Louis Armstrong who established singing as a distinct art form in jazz, realising that a singer could improvise in the same manner as instrumentalist, and establishing scat singing as a central pillar of the jazz vocal art.
Jazz musicians Don Redman, Cliff Edwards, and Red Nichols all recorded examples of scat earlier than Armstrong. However, the record Heebie Jeebies and subsequent Armstrong recordings introduced scat singing to a wider audience and did much to popularize the style. Armstrong was an innovative singer who whilst experimenting with all kinds of sound, improvised with his voice as he did on his instrument. In one famous example, Armstrong scatted a passage on I'm A Ding Dong Daddy From Dumas – he sings "I've done forgot the words!" in the middle of recording before taking off in scat.
The entrance of Billie Holiday into the world of jazz singing in the early 1930s was a revelation. She approached the voice from a radical angle, explaining, in her own words,
I don't feel like I'm singing, I feel like I'm playing the horn.
Compared to other great jazz singers, Holiday had a rather limited vocal range of just over an octave. Where Holiday's genius lay, however, was to compensate for this shortcoming, with impeccable timing, nuanced phrasing, and emotional immediacy, qualities admired by a young Frank Sinatra.
With the end of prohibition in the United States, a more 'danceable' form of jazz music arose, giving birth to the 'Swing Era', and with it big bands such as those led by Count Basie, Benny Goodman, Duke Ellington, Tommy Dorsey, Jimmie Lunceford, Glenn Miller, Artie Shaw and Chick Webb. Many of the great post war jazz singers sang with these bands in the infancy of their careers.
With the end of the 'Swing Era', the great touring Big bands of the past decade were no longer a viable option, and the demise of the typical big band singer was further complicated by the advent of be-bop as a creative force in jazz.