Side altar

A side-altar or bye-altar is an altar that is subordinate to the central or high altar in a church. The term is generally applied to altars situated in bays of the nave, transepts, etc.^[1] Side-altars may be recessed in a side-chapel, or simply built against a main aisle wall.

A bye-altar in the Kapucijnenkerk; Ostend, Belgium. Notice the high altar in the background.

Before the liturgical reforms arising from the Second Vatican Council, separate Masses may be celebrated simultaneously by other priests at side-altars, even as there is an ongoing Mass at the high altar.