In a saddle stitched booklet the bulk of the paper causes
the inner pages to extend further out than the outer pages when folded.
When trimmed the inner pages are narrower than the outer pages.
The illustration above shows an exaggerated view of how the inner
pages of a saddle stitched booklet "creep" out and extend
beyond the edge of the outer pages when folded.
Creep Allowance: Adjusting the page layout of inner
spreads to maintain a constant outer margin when the saddle stitched
booklet is trimmed.
Problem: Creep varies depending on the thickness of the
paper and the number of pages. If there is no creep allowance, when
pages are trimmed the outer margins become narrower toward the center
of the booklet and there is the possibility that text or images may
be cut off.
Solution: If creep is noticeable, copy can be repositioned
toward the center of the spread for those pages in the center of the
booklet. When trimmed, all pages will have the same outer margins
and no text or images are lost. Some page layout software can automatically
calculate and adjust layouts for creep -- moving the page elements
of affected spreads in small increments.
The illustration below shows how page elements are adjusted in small
increments over a series of spreads.