Ever wonder why that hook on the end of a tape measure isn't fixed in position? It wiggles on its rivets to make up for the hook's thickness. Push the tip against something for an inside measurement, and the hook is included in the reading; hook it on the end of a piece for an outside measure, and the resulting gap equals the space the hook would take up. But if the hook leaves you guessing when you're measuring short distances, line up the 1-inch mark at the start, then deduct that extra inch from the final measurement.