A more thorough explanation:

A diptych is a set of two tablets that are usually made of wood or metal and tied together with string through holes at the edges so that they can fold over like a book with two leaves. The word "diptych" comes from the Greek word "diptycha," which means "two-leaved."

In Roman law, diptychs were often used to send letters. The text was sometimes written using a stylus, once on the inside waxed leaves and again on the outside, so that it could be read without opening the tablets.

In Ecclesiastical law, diptychs were used by the church to register names of those making supplication, and to record births, marriages, and deaths. The registry of those names was also called a diptych.

The recitation of the name of any prelate or civil ruler in the diptychs was a recognition of his orthodoxy; its omission, the reverse. The mention of a person after death recognized him as having died in the communion of the church, and the introduction of his name into the list of saints or martyrs constituted canonization.

For example, in the early Christian church, diptychs were used in the celebration of divine worship. They were placed on the pulpits or reading desks, which may still be seen in ancient basilicas at the west end of the choir or presbytery. From them, the names of the celebrating priests, those who occupied the superior positions in the Christian hierarchy, the saints, martyrs, and confessors, and, in process of time, also of those who had died in the faith were read to the congregation of the faithful.

The use of diptychs died out in the Western Church between the ninth and twelfth centuries, but it still continues in the Eastern Church.