The contents of books are also called books, as are other compositions of that length. For instance, Aristotle's Physics, the constituent sections of the Bible, and even the Egyptian Book of the Dead are called books independently of their physical form. Conversely, some long literary compositions are divided into books of varying sizes,
which typically do not correspond to physically bound units. This
tradition derives from ancient scroll formats, where long works needed
several scrolls. Where very long books in codex format still need to be
physically divided, the term volume is now normally used. Books may be distributed in electronic form as e-books and other formats. A UNESCO
conference in 1964 attempted to define a book for library purposes as
"a non-periodical printed publication of at least forty-nine pages,
exclusive of cover pages".[2] A single sheet within a codex book is a leaf, and each side of a leaf is a page. Writing or images can be printed or drawn on a book's pages.