Readers used to perusing typical physical literature are probably wondering why there is so much white space surrounding the text. For example, on A4 paper a document will typically have 44 mm margin widths on the left and right of the page, leaving about 60% of the page width for text. The reason is improved readability. Studies have shown   that it's easier to read text when there are 60–70 characters per line—and it would seem that 66 is the optimal number. Therefore, the page margins are set to ensure optimal readability, and excessive margin white space is tolerated as a consequence. Sometimes, this white space is left in the inner margin with the assumption that the document will be bound.