From the middle of the fourth century down to the end of the sixteenth we have an unbroken chain of examples still existing. Individual pieces may, perhaps, in many instances be of questionable origin as regards the country of the artist, and, sometimes, with respect to the exact date within fifty or even a hundred years. But there is no doubt whatever that, increasing in number as they come nearer to the middle ages, we can refer to carved ivories of every century preserved in museums in England and abroad. Their importance with reference to the history of art cannot be overrated. There is no such continuous chain in manuscripts, or mosaics, or gems, or textiles, or porcelain, or enamels. Perhaps, with the exception of manuscripts, there never was in any of these classes so large a number executed nor the demand for them so great. The material itself or the decorations by which other works were surrounded very probably tempted people to destroy them; and we may thank the valueless character of many a piece of carved ivory, except as a work of art, for its preservation to our own days. The most important ivories before the seventh century are the consular diptychs. The earliest which still exists claims to be of the middle of the third century, the latest belongs to the middle of the sixth. Anything doubled, or doubly folded, is a diptych: δίπτυχον; but the term was chiefly applied to the tablets used for writing on with metallic or ivory styles by the ancients. When these tablets had three leaves they were called triptychs, and of five or more leaves pentaptychs or polyptychs. Inside, each leaf was slightly sunk with a narrow raised margin in order to hold wax; outside, they were ornamented with carvings. They were not always of ivory; frequently of citron or of some less costly wood, and for common use were probably of small size, convenient for the hand and for carrying about.