If you need to get a measurement for a difficult piece too big for your normal calipers, a bar clamp can work in a pinch.
During a recent fireplace mantel rebuild, I needed to take an accurate inside measurement of an assembled frame in order to replace a raised panel. The existing panel, and the rounded-over edges of its opening, made it difficult to take the reading with my tape measure. Instead, I grabbed a bar clamp and used the flares on the bottom of the jaws to serve as a quick-and-dirty caliper. I spread the jaws open until the outer edges of the flares contacted the panel opening. Then I carefully removed the clamp and measured the flares for an accurate reading of the opening. It was just the tool I needed!
