The answer
On top of a Coptic altar, there is a rectangular slot carved out, usually about 2.5 cm deep. In this slot, a consecrated altar board is placed, made of timber, or more rarely of marble. On the slate, one or more crosses are engraved, along with the first and last letters of the Greek alphabet: alpha and omega — α and ω. Sometimes a verse from the book of Psalms is engraved such as: “His foundations are in the holy mountains. The Lord loves the gates of Zion, more than all the dwellings of Jacob. Glorious things are spoken of you, O City of God!” (Ps. 87:1–3).
It is highly likely that altar boards date back to the times of persecution when the faithful needed to easily carry it around. However, others believe that they were created to replace portable altars.
Fr. Tadros Yacoub Malaty