Creep is the shifting position of the page in a saddle-stitched bind. Creep moves the inside pages away from the spine.
All books and brochures use a sheet that is folded to create separate page areas. Because the sheet itself has some thickness, each fold causes a very small incremental shift in the location of the edge of each page relative to the others. The result is that the edges of pages furthest from the saddle's innermost fold seem to move away from the edges of pages closest to the innermost fold. As the number of pages in the saddle increases, so does the amount of this shift. To counteract the effects of this shift, the bindery trims the edges of the finished book to a common edge.
Trimming the bound pages resolves only part of the problem, however, as the edges of pages move, so do the content areas imaged on them. The apparent shift of content area caused by folding a sheet multiple times is known as creep or binder's creep. The effect of creep is that the content area of pages closer to the innermost fold in a saddle appear to move towards the outer margins of the page.
Creep becomes more noticeable as the number of sheets in a saddle increases, and its effect is most noticeable on pages farthest from the center fold of the saddle. Consequently, a book layout must also specify the number of sheets in each saddle because the number of sheets in each saddle affects the amount of creep that needs to be accounted for.