All mounts are sized by the height of the stamp, not the height of the mount. These measurements are in millimeters. So, if you have a stamp that is 36mm high, you would order the 36mm mount strip (in this case stock #928). That mount is made to hold a 36mm high stamp. If you had a stamp that is 38mm high, you would order the 39mm mount (since there is no 38mm size). Always order the size mount that matches your stamp height. If there is no match, then order the next size up. When you receive your first order from us, we include is a millimeter gauge which is part of our mount order form.