Particular interest attaches to one block book called the Speculum Humanae Salvationis or "Mirror of Salvation." In a way this book is the connecting link between block books and type printed books. There is no copy of this book in existence printed entirely from wooden blocks. Most of the early editions are printed from movable types with a block printed illustration at the head of each page. One of them, however, has twenty pages of the text out of the sixty-two which constituted the entire book printed from wood blocks. These twenty pages are inserted at intervals among the others, and how and why they came there is a riddle beyond guessing. It has been conjectured by some that the long-held belief that Gutenberg was a polisher of mirrors is erroneous and that the reference in certain of the scanty documents concerning him to business about mirrors may refer to attempts on his part to print an edition of this book, "The Mirror of Salvation." In making a block book the design was cut on the side of a flat piece of wood, not on the end of the block as was the later practice in wood engraving. Sometimes, as has been said, a design thus cut was only a picture. Sometimes it was both picture and text. The design was cut in relief, that is to say the wood was cut away leaving the design to be impressed upon the paper raised. The block was then thoroughly wetted with a thin, watery, pale brown material much resembling distemper. A sheet of damp paper was laid on it and the back of the paper was carefully rubbed with a dabber or burnisher. It is probable that other inks were employed, especially for vellum, and it is also extremely probable that a rude press, ancestor of the modern printing press, was used to produce the impressions in many cases. The resulting book consisted of sheets printed on one side only, although there are a few very late examples in which printing appears on both sides. The pictures were commonly roughly colored by hand. Playing cards were at one time supposed to have been the first products of this method of printing. It was naturally supposed that the small and comparatively simple design on the face of the playing card might be regarded as the original from which the more elaborate picture and book might be developed. This opinion has now, however, been abandoned, as it is known that the earliest playing cards were hand drawn and painted and that the block printed playing cards which we have date from the 15th century when block printing was very common. It has already been said that these blocks contained not only pictures but text, one very important block book consisting of text alone. What determined the form of the letters composing this text?