In Europe, before the invention of the printing press, books were copied by hand, originally onto vellum and later onto paper. Paper was expensive and vellum was much more expensive. A single book cost as much as a house. Books, therefore, were long-term investments expected to be handed down to succeeding generations. Readers commonly wrote notes in the margins of books in order to enhance the understanding of later readers. Of the fifty-two extant manuscript copies of Lucretius' "De rerum natura" (On the Nature of Things) available to scholars, all but three contain marginal notes.