Smart woodworkers know that the less you use your tape measure or rule, the fewer mistakes you make. That’s one good reason to use a bar gauge instead. A bar gauge is simply an adjustable length stick, and Veritas makes some hardware that makes it easy to make your own.
In the Great White North, formally known as Canada, Veritas makes the bar gauge heads from brass and ABS plastic. To use them you just need two 1/4″ by 3/4″ strips, which are easily made by ripping 3/4″ thick board. Slip the bar gauge heads over the two sticks and tighten them down when the ends are at the desired length. Now you can easily transfer or compare measurements.
If you buy from Lee Valley, you also get two brass pins that you can use for transferring inside and outside measurements. You’ll pay $15 for these bar gauge heads before shipping.