Bookbinding - the distance by which the cover boards of a hardback binding extend beyond the book block. Also called the overhang.

 |  book block  |

The asterisks are just below and just above the squares.

The square is usually 1 1/2 to 2 times the thickness of the cover board. Expressing square size in terms of board thickness means that books of different sizes will always be in proportion.

The extra extent of cover serves to protect the pages of the book block from wear and damage. However, it also poses problems in book construction. Unless the book block is a sufficiently solid unit, it will tend to sag down until it rests on the bookshelf, slowly tearing the binding apart.

European bookbindings have included a square from the earliest examples we have. However, the Islamic tradition of bookbinding uses covers even with the book blocks.