Prothesis (altar)

[1][2] During the reign of the Emperor Justin II (565–574), it came to occupy its own separate chamber to the north of the sanctuary, having a separate apse, and joined to the altar by an arched opening.

So that from this time forward, large Orthodox Churches were triapsidal (having three apses on the eastern side).

[citation needed] Smaller churches still have only one chamber containing the altar, the prothesis and the diaconicon.

In the Syriac Churches, the ritual is different, as both prothesis and diaconicon are generally rectangular, and the former constitutes a chamber for the deposit of offerings by the faithful.

[citation needed] In the Coptic Orthodox Church, the men will enter the prothesis to receive Holy Communion (the women receive in front of the holy doors), and must remove their shoes before entering.

The triple apse of an Orthodox Church. The altar is in the larger central apse, the prothesis in the apse to the right, and the diaconicon in the one to the left.
Sacred vessels used in the Orthodox Divine Liturgy (photograph by Sergey Prokudin-Gorsky , 1911)