I. Archaeology. Etymology reveals its form: di.j ptu,ssein, to fold twice. A diptych is essentially two tablets or valves joined so as to fold together. From the start it was a deluxe variant on the ordinary wax tablet; the inner faces, smooth and wax-covered, were used for writing, the outer were often decorated in relief or embossed. Wide diffusion is attested in the later imperial age, when it became more and more common for ordinary consuls to give writing tablets as gifts to illustrious persons; the custom should also be referred to the more restricted circle of the emperor. This characteristic, in this particular period, made the diptych a useful vehicle for understanding court art, heavily influenced by the classicism and hence conservatism expressed by and for the ruling class. Having become an instrument of political propaganda, its use was limited by a regulation of 384 CTh XV, 9,1, which allowed the right to possess diptychs only to those of consular rank; this is indirect evidence of their wide diffusion in the course of the 4th c. Generally speaking, we can divide diptychs into consular and private: the former are easily recognizable, since the consul appears on the outside of the valves, shown with the attributes of his dignity dress, insignia, sella curulis, pose, etc.; the latter, with various subjects on the valves, were often shown to belong to great private familiae by inscriptions on a scroll. Both these types might pass into the third category, that of ecclesiastical diptychs, which could either be given to the church or made expressly by or for it. One of the most celebrated examples is that of Nicomachi and the Symmachi, dated late 4th c.: it is classicizing in the profiles of the images depicted, and simultaneously pictorial following contemporary taste in the details of the subsidiary decoration, informed by meticulous style and negative relief. More precisely datable are those of Anicius Probus, consul in 406, and Felix, consul in 420, where the pictorial aims of contemporary plastic art, very evident in the diptychs of Basil 480 and Boethius 487, are more clearly seen. In the 6th c. output does not seem to decline in relation to the augmented prestige of certain familiae of noble rank. Numerous replicas of the diptychs of Areobindus 506, scattered in various collections, reveal behind the formal aspect linked to the artistic milieu of Constantinople the vehicles of Byzantine propaganda in the West.