Cathedral design
Most cathedrals in Western religions are built in the shape of a cross. The main entrance is at the west end, at the bottom of the cross. A long central aisle, called the nave, and two side aisles extend from the entrance. The two arms of the cross, called the transepts, meet the nave at the crossing. Worshipers assemble in the nave and transepts.
The altar and the seats for the choir lie at the east end of the nave, in front of a semicircular area called the apse. A walkway called the ambulatory extends around the apse and may open onto several chapels. The bishop's throne usually stands to one side of the altar.
Excerpt adapted from the "Cathedral" article, The World Book Encyclopedia © 1999