1. Planar sheet, comprising: on a first surface of the sheet, at least one printed mark; on a second surface of the sheet, opposite said first surface, at least one invisible printed mark vertically adjacent to the printed mark on said first surface; in at least one pair of vertically adjacent marks formed on said opposite surfaces, at least one of the marks of said pair has unique content; and wherein a content of said printed mark on said first surface is representative of a position of said invisible mark on said second surface of the sheet.