Millions of tape measures carry mysterious black diamonds; they help builders position floor joists without repeated calculations

A small black diamond labeled on the tape measure indicates a precise 19.2 inches for floor joist spacing, effectively coordinating with plywood sheet dimensions. This alignment provides proper support for panel edges resting on joists. Span tables released by engineered wood associations highlight this spacing option specifically. It's important to differentiate this from wall stud spacing, which adheres to separate guidelines.