what is the diamond on a tape measure

The diamond on a tape measure is a stud or joist mark, which is used to space I-beam "timbers" in new construction. The black diamonds on metal tape measures appear every 19 3/16 inches, which is the spacing required for engineered floor joists. On a UK tape measure, the diamond is usually found every 19.2 inches along the tape, which is the spacing for joists in UK structures. The diamond marks on tape rule blades are for spacing these engineered floor joists in new construction. The diamond-shaped markings are used to indicate where to place studs or joists in a wall or floor.