"The earliest treasure bookcovers can be divided into those made of ivory, and those made of precious metals. The ivory covers found their direct models in the diptychs of the late Empire. These diptychs, luxurious versions of the traditional Roman wax writing tables in hinged pairs, were distributed as gifts by various Roman high officials to commemorate their entries into office. Ivory diptychs are first mentioned in a sumptuary edict of 384, enacting that ivory might be used for the diptychs only of the two annual consules ordinarii, whose assumption of office on 1 January (though their once-powerful title was now purely honorary) inaugurated the civil year. Because of the division of the Empire, consuls were elected in pairs both in Rome and Constantinople, and so their diptychs were manufactured in both cities. Until the extinction of the consular office, in 534 in Rome and 541 in Constantinople, many thousands of consular diptychs must have been created, presumably in workshops under the direction of the Imperial scrinia, or chancery. Those surviving, less than a hundred, mostly owe their preservation to their reuse in the Middle Ages as decorations for bookcovers.
"The earliest ivory plaques made explicitly as bookcovers rather than as diptychs or casket pieces are probably a famous pair in the Cathedral Treasury of Milan. Their layout is precisely that of the most luxurious consular diptychs, those meant for presentation to the emperor himself. But in place of Imperial symbolism, the panels are covered with scenes from the lives of Christ and Mary, together with the evangelist symbols and portraits. The center panels of each cover bear respectively an Agnus Dei and a cross, worked in silver-gilt and stones and attached to the ivory. The covers must have been made for a deluxe, large-format Gospels codex, now missing. They have been dated to the second half of the fifth century, and they come from the Western Empire, but have not been more precisely localized" (Needham, Twelve Centuries of Bookbinding 400-1600  21-22).