With this handy gadget, it’s easy to determine the size of almost any British Standard size O-ring. Section size is identified by using the slots at the top. The BS size number of small O-rings can be found by matching them with the silhouettes along the sides of the gauge. Larger O-rings are placed around the two halves of the appropriate spindles at the top of the gauge. The slide is then extended until it is taut, and the BS size number is shown in one of the small windows near the top of the main body.