Prothesis (Gr. irp??anc, a setting forth, from irporeO~vae, to set forward or before), in the liturgy of the Eastern Orthodox Church, is the name given to the act of setting forth the oblation , i.e. the arranging of the bread on the paten , the signing of the cross ( pwyL~etv) on the bread with the sacred spear , the mixing of the chalice, and the veiling of the paten and chalice (see F. E. Brightman , Liturgies Eastern and Western, 1896).

The term is also used, architecturally, for the place in which this ceremony takes place, a chamber on the north side of the central apse in a Greek church, with a small table.

During the reign of Justin II (565-574) this chamber was located in an apse, and another apse was added on the south side for the diaconicon , so that from his time the Greek church was triapsal. In the churches in central Syria the ritual was apparently not the same, as both prothesis and diaconica are generally rectangular, and the former, according to De Vogue , constituted a chamber for the deposit of offerings by the faithful. Consequently it is sometimes placed on the south side, if when so placed it was more accessible to the pilgrims. There is always a much wider doorway to the prothesis than to the diaconicon, and there are cases where a side doorway from the central apse leads direct to the diaconicon, but never to the prothesis.