What does the diamond mark on tape measures at 19 and 3/16 inch and at 38 and 3/8 inch, 57 5/8inch etc mean? Tom Silva explained it on one of the shows and I said to myself write that down but of course I didn't.
its for laying out engineered floor joists, such as TJI's (wood i joists) and ojt's (open truss joists). the can support more weight than conventional lumber so they can be spaced farther apart, and can have greater spans
typically with 2x stock they can be spaced no more than 12" apart