Late in the 19th century a new writing material made of wood or other flexible fibre treated with chemicals and loaded with clay was invented, to which we also give the name paper. This new material has almost entirely driven the old rag paper out of the field and is now the paper of commerce. Much of this material is far inferior to rag paper. The inferior qualities of it, at any rate, lack durability even when not exposed to wear. It is good enough for the great number of uses where permanence is not required. It should only be used for books of permanent value, especially for records and historical material, when there can be no doubt of the care used in the manufacture and the quality of the fibre employed. A 15th-century book on rag paper is as good to-day as the day it was printed. Most of the paper now in use possesses no such lasting qualities. In addition to these three leading materials, much use has been made of tablets (Latin _tabella_). The commonest form of tablet was a thin board with one or both sides slightly cut away in such a way as to leave a narrow rim all around. The shallow depression inside this rim was then filled with wax sufficiently stiff to hold its position in ordinary temperatures but sufficiently soft to be easily marked with a sharp instrument called a stylus. The writing could be easily erased by rubbing with a hard smooth object, perhaps a ball at the reverse end of the stylus, and the wax was then ready for another impression. Sometimes these tablets were made of wood covered with paint or a composition from which the writing could be easily washed off. This was the prototype of the schoolboy's slate of to-day and was used for the same purpose. While tablets were ordinarily used for writing of a purely temporary nature, they were occasionally used for permanent records and especially for correspondence. Two or more tablets could be put together with the wooden sides out, bound, and sealed. In this way the writing was secure from observation or interference and the tablets were less liable to injury than papyrus or vellum. Tablets were used at a very early period and continued to be used, especially for correspondence, all through the middle ages and into the 16th century. Sometimes a considerable number of them would be fastened with thongs by one edge so as to form a continuous document which was one of the precursors of the modern book. The British Museum has a document of this sort consisting of nine leaves about 7 x 9 inches. The writing on it is in shorthand, which is by no means a modern contrivance. This particular document is of Greek origin and dates from about the 3d century A.D.