A bar gauge has two main uses, firstly for comparing measurements such as the diagonals in a drawer and secondly for transferring measurements for fitting shelves into recesses and the like. The openings in the heads are 3/4" x 1/2" and are designed to take two pieces of timber 3/4" x 1/4" (not supplied) of any convenient length. This is one of those really simple tools which you wonder how you ever managed without.