Cantoris is the side of a church choir occupied by the Cantor

Cantor (church)

A cantor is the chief singer employed in a church with responsibilities for the ecclesiastical choir; also called the precentor....

. In English churches this is typically the choir stalls on the north side of the chancel

Chancel

In church architecture, the chancel is the space around the altar in the sanctuary at the liturgical east end of a traditional Christian church building...

, although there are some notable exceptions, such as Durham Cathedral

Durham Cathedral

The Cathedral Church of Christ, Blessed Mary the Virgin and St Cuthbert of Durham is a cathedral in the city of Durham, England, the seat of the Anglican Bishop of Durham. The Bishopric dates from 995, with the present cathedral being founded in AD 1093...

 and Southwell Minster

Southwell Minster

Southwell Minster is a minster and cathedral, in Southwell, Nottinghamshire, England. It is six miles away from Newark-on-Trent and thirteen miles from Mansfield. It is the seat of the Bishop of Southwell and Nottingham and the Diocese of Southwell and Nottingham.It is considered an outstanding...

. The opposite side is known as Decani

Decani

Decani is the side of a church choir occupied by the Dean. In English churches this is typically the choir stalls on the south side of the chancel, although there are some notable exceptions, such as Durham Cathedral and Southwell Minster...

.

From the perspective of the congregation facing the altar, which by convention is regarded as liturgical East, this would be on the LEFT (liturgical North) side.

While the cantoris side of the choir corresponds to the Gospel side of the altar (so called from the custom of reading the Epistle from the south end of the altar, and the Gospel from the north end of the altar), cantoris and decani properly refer only to sides of the choir, not to the sides of the altar.

If the choir voices divide in a piece of music, the Cantoris side would usually sing the second or lower parts.