This term indicates a support used for writing consisting of two symmetrical tablets, called leaves, held together by a hinge, originally made of wood or metal. Their inner surface was covered with wax, a material on which it was possible to write with a stylus.
The diptych, which resembled a rudimentary book, was shut with ribbons or strings. In the 4th century AD, diptychs became small works of art: the decorations, at first placed only on the outside of the leaves, extended to the inside, while the hinges became more and more precious and worked with ivory or noble metal.