Transepts

A transept (with two semitransepts) is a transverse part of any building, which lies across the main body of the edifice. In churches, a transept is an area set crosswise to the nave in a cruciform ("cross-shaped") building within the Romanesque and Gothic Christian church architectural traditions. Each half of a transept is known as a semitransept.

The transept of a church separates the nave from the sanctuary, apse, choir, chevet, presbytery or chancel. The transepts cross the nave at the crossing, which belongs equally to the main nave axis and to the transept. Upon its four piers, the crossing may support a spire (e.g., Salisbury Cathedral), a central tower (e.g., Gloucester Cathedral) or a crossing dome (e.g., St Paul's Cathedral). Since the altar is usually located at the east end of a church, a transept extends to the north and south. The north and south end walls often hold decorated windows of stained glass, such as rose windows, in stone tracery.

...
Wikipedia