Vertical lines used to indicate the center of a 2-page spread for folding or cutting are the center marks.
Center marks usually appear at the top and bottom edge of the spread. Center marks are printed with thin lines of about 1/4 to 1/2 inches in length. Because they appear at the point where a spread is folded (or cut) they are not usually noticeable in the finished book or newsletter.
Crop marks serve a similar purpose.
Also Known As: fold marks