Flats depths on subs may be measured in two different ways. The first, and most common, is by the depth of the flat from the diameter of the sub (1), how much material is removed. The second method of measurement is to measure the distance between the flat surface to flat surface (2), or the opening of the pipe-handling tool. If the flat has a taper, please give us the length at the top and again at the bottom of the flat.