Block books, also called xylographica, are short books of up to 50 leaves, printed in Europe in the second half of the 15th century as woodcuts with blocks carved to include both text and illustrations. The content of the books was nearly always religious, aimed at a popular audience, and a few titles were often reprinted in several editions using new woodcuts. Although many had believed that block books preceded Gutenberg's invention of movable type in the first part of the 1450s, it now is accepted that most of the surviving block books were printed in the 1460s or later, and that the earliest surviving one may date to about 1451. They seem to have functioned as a cheap popular alternative to the printed book, which was still very expensive at this stage. Single-leaf woodcuts from the preceding decades often included passages of text with prayers, indulgences and other material; the block book was an extension of this form. Block books are very rare, some editions surviving only in fragments, and many probably not surviving at all.