Normally, eye glass frames have three numbers which includes a horizontal measurement across the lens. However, if you wear bi-focals and especially tri-focals, it is a good idea to ask for the vertical measurement of the lens when ordering frames on-line. The new styles are rather narrow and some simply will not accommodate a tri-focal lens, especially if you order "executive" tri-focals (which are becoming harder to find). While this will limit some of your selections, it is worth checking into rather than finding out after the fact that the lens is too small to make your tri-focals effective. I have one pair that I was assured would be OK for tri-focals, but the very bottom part of the lens (what I use for reading) is so small that I can barely read for any length of time. My neck hurts and my eyes become strained; just not good. A larger lens in my older glasses did not cause this problem.