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The apse is the eastern part of the altar in the form of a semicircle or polygon, covered by a half-dome or closed semi-arch (conch). In a three-part altar, it may be designated for the altar itself, for the vestment room and for the sacrificial table
Source: "SP 31-103-99. Buildings, Structures and Complexes of Orthodox Churches"
(adopted by the Decree of the State Construction Committee of Russia dated December 27, 1999 No. 92)