Easy to use U.S.S. & S.A.E. plastic nut and bolt diameter and length measuring gauge. Just push the bolts through the holes to find the bolt diameter. The length gauge measures regular heads from the left end and countersunk heads from the notch at the right end. Nuts drop on the plugs along the top edge of the gauge. S.A.E. fine thread nuts fit to the base of the plugs, U.S.S. coarse thread nuts fit half-way.