Select the letter that matches your shoe size range. Letters on all Superfeet products represent heel cup width. For Superfeet insoles to work properly, your heels must be "in" the heel cup entirely, not "on" the heel cup on either side of foot. Stand on a pair of Superfeet in the letter that corresponds closest to your shoe size making sure your heel fits nice and snug "in" the heel cup. If that heel cup is too small, select the next letter size up. After you find your perfect heel-cup fit, simply trim the excess foam (using the sock liner from any of your shoes as a guide) and insert the new trimmed insoles into your shoes (removing the sock liners first).