From Greek διακονικóν (service room), one of the side rooms found frequently in the early Christian basilica, serving many of the functions of today's sacristy in the Latin Church. The diaconicum was more common in the Christian East and was generally located just south of the apse. Although it served principally as a vesting place and "treasury" (σκευοφυλάκιον or κειμηλιαρχε[symbol omitted]ον) where the liturgical books and vessels, as well as relics, might be kept, it served also as a place for preparing the bread and wine and for reserving the Eucharist (whence its other names, πρóθεσις and μαστοφóριον). Since the 7th century, the two functions were differentiated, and two rooms, the prothesis and the diaconicum, balance one another on either side of the apse of church buildings in the Christian East.
Bibliography: g. bandmann, "Über Pastophorien und verwandte Nebenräume im mittelalterlichen Kirchenbau," Kunstgeschichtliche Studien für Hans Kauffmann, ed. w. braunfels (Berlin 1956) 19–58.
[t. f. mathews/eds.]