E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A register folded into two leaves, opening like our books, and not like the ancient scrolls. The Romans kept in a book of this sort the names of their magistrates, and the Roman Catholics employed the word for the registers in which were written the names of those bishops, saints, and martyrs who were to be specially commemorated when oblations were made for the dead. (Greek, diptuchos, folded in two.)
The Greeks executed small works of great elegance, as may be seen in the diptychs, or ivory covers to consular records, or sacred volumes used in the church service.T. Flaxman: Lectures on Sculpture, iii. p. 98.