What are crossovers?
A term referring to a two-page spread in which an image crosses over onto both pages of a booklet or catalog. It is when the text or images crosses over the middle spine from the left hand page to the right hand page. With this practice there can be misalignment across the spine. To avoid flawed appearances with a crossover avoid putting focal points and text (especially small text) near the spine or middle of the artwork.
Some bindings (saddle stitching, specifically) require special instruction for crossover images.