what is the diamond on a tape measure for

The black diamond on a tape measure is a stud or joist mark. It is used for spacing I-beam "timbers" in new construction. The diamond marks on tape rule blades are for spacing these engineered floor joists because they can support more weight than their dimensional lumber counterpart, so they often have different spacing requirements. The diamond is typically found every 19.2 inches along the tape in the UK, but in the US, some tape measures also include a diamond at 16-inch intervals. The diamond is intended to be used when you need to make a measurement marking, but dont have a pencil or chalk to hand.