Creep (also called Shingling, Thrust or Push-Out) refers to the effect of pages of a book being pushed out slightly once the pages are gathered together for binding on a Saddle-Stitched publication. Creep can also occur within signatures of a burst-bound book.
Creep is normally trimmed off of the book afterwards, but this can result in folios and design elements being trimmed off of the finished publication. To prevent this from happening, imposition programs such as Creo Preps counter this by pushing the pages slightly into the centre as the pages get closer to the centre of the book.
An unfortunate side effect of compensating for creep is that the solution tends to impact on crossovers (objects which extend across both pages of a two-page reader’s spread), and is most visible on a centre spread as the crossover has a visible ‘split’ in the middle of it.
If using crossovers in the artwork, make sure the artwork on the outside margins of the book will compensate for creep, such as making sure folios are far enough away from the outside edges of the book. Similarly, If the outside margins will be influenced by creep, think seriously about refraining from the use of crossovers, particularly in the centre spread of the book. If all features must co-exist, make sure that the printer is supplied native files so that they may make any adjustments as necessary to compensate for the creep manually.