Plate XI · Apse (from Greek ἀφίς "Joint" and later also "Arch, Vault"), or Exedra
Vol. I (1948) · full-page plateAbove: APSE OF S. APOLLINARE IN CLASSE (6th cent.) - Ravenna. Below: APSE OF S. GIOVANNI A PORTA LATINA (8th cent.) - Rome.
Enciclopedia Cattolica