A basic unit in book design is the page spread. The left page (called verso) and right page (called recto) are of the same size and aspect ratio,

and are centered on the gutter where they are bound together at the spine.

The design of each individual page, on the other hand, is governed by the canons of page construction.

The possible layout of the sets of letters of the alphabet, or words, on a page is determined by the so-called print space, and is also an element in the design of the page of the book. Clearly, there must be sufficient space, at the spine of the book, if the text is to be visible. On the other hand, the other three margins of the page, which frame the book, are made of the appropriate size for both practical and aesthetic reasons.