The choir was simply the east part of the nave, and was fenced off by a screen or low railing, called cancelli, which is where the English word chancel comes from.
Isidore of Seville and Honorius of Autun write that the term is derived from the "corona", the circle of clergy or singers who surrounded the altar.
When first introduced, the choir was attached to the bema, the elevated platform in the centre of the nave on which were placed seats for the clergy and a lectern for scripture readings.
The architectural details of the choir developed in response to its function as the place where the Divine Office was chanted by the monastic brotherhood or the chapter of canons.
Some cathedrals have a retro-choir behind the high altar, opening eastward towards the chapels (chantries) in the eastern extremity.
After the Reformation Protestant churches generally moved the altar (now often called the communion table) forward, typically to the front of the chancel, and often used lay choirs who were placed in a gallery at the west end.
With the emphasis on sermons, and their audibility, some churches simply converted their chancels to seat part of the congregation.
The upper part of the monk's stall is so shaped as to provide a headrest while sitting, and arm rests when standing.