In church architecture, sedilia (plural of Latin sedīle, “seat”) are seats, usually made of stone, found on the liturgical south side of an altar, often in the chancel, for use during Mass for the officiating priest and his assistants, the deacon and sub-deacon. The seat is often set back into the main wall of the church itself. The custom of recessing them in the thickness of the wall began about the end of the 12th century; some early examples consist only of stone benches,
A piscina is a shallow basin placed near the altar of a church, or else in the vestry or sacristy, used for washing the communion vessels.
The sedilia and piscina in St. Peter’s were originally 12th century, but were restored by the Victorians in the 19th century.