At first these images appeared upon single sheets bearing only the picture. Later, words were carved upon the block in addition to the picture, giving us on one page the combination of picture and text. Very probably the owner could not read the inscription, but he could be told what it meant and the very form of it would recall to his mind the words which he had learned. From this single picture there were two lines of development. One was the binding up of several pages of such pictures and texts into a book. Each page was printed from a single block and the result was a block book. Of these there were two especially famous and often printed, The Biblia Pauperum, or Poor Man’s Bible, and the Speculum Humani Salvationis, or Mirror of Human Salvation. The Poor Man’s Bible, of course, was a collection of prints recalling the biblical stories. The Mirror went further afield and showed pictures representing moral teaching. Great numbers of these books were printed and circulated, especially in Holland and Germany. The other development was the printing of whole pages of text. Certain books were in great demand. The advantage of being able to reproduce them quickly and cheaply was obvious. The two best known are alphabet books for beginners called Abecederia, or Abecederium in the singular, and the elementary Latin grammars abstracted from the works of an old Latin grammarian named Donatus. These grammars were commonly called Donatuses. These books, especially the school books, were printed mostly on vellum, partly because of its greater durability and partly because the use of paper was not yet common. We have here the lines of development which led directly to the invention of typography. We have already mentioned books printed from plates with pictures without text, pictures with text, and text without pictures. What would be more natural than to cut off the part of the block containing the text and use the picture alone, or to combine the text with text cut from another block or with another picture from which the text had been removed? If we could do this why could we not cut out a single word or a single letter and why not make a considerable number of these single letters and combine them into words? If parts of two or more blocks were to be used for the same picture, they must be fastened together in some way, or as we should say today the form must be locked up. Why could not separate letters be fastened together in the same way so that we could print anything we wanted by the simple process of putting together the necessary letters in the proper relation? As we shall see presently, this is exactly what happened, and the invention of printing thus considered is the most natural thing in the world.