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

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 Cantoris

Cantoris

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

.

In English churches, in the typical arrangement, looking from the altar into the congregation, this would be the RIGHT (liturgical South) side.

If the choir voices divide in a piece of music, the Decani side would usually sing the first or higher parts.