A book is a series of pages assembled for easy portability and reading, as well as the composition contained in it. The book's most common modern form is that of a codex volume consisting of rectangular paper pages bound on one side, with a heavier cover and spine, so that it can fan open for reading. Books have taken other forms, such as scrolls, leaves on a string, or strips tied together; and the pages have been of parchment, vellum, papyrus, bamboo slips, palm leaves, silk, wood, and other materials.