Back in the days of going into stores to try things on, you could get away with not knowing your size. But now that more and more of us are buying clothing and accoutrements on the internet, there's a bunch of numbers that define us as our own uniquely-sized physical entity.

Our measurements for things like footwear, pants, and hats are easy to come by, using that foot-measuring thingy (actually called a Brannock Device, after the inventor) and a flexible tape measure for your waist and head. (Interesting tidbit--if you've ever wondered why men's hat sizes have numbers like 7 1/8 and women's hats have sizes like 22 1/2, it's because men's hats are by head diameter, while women's hats go by circumference. Go figure.)

But did you ever wonder what the metric is for getting a properly-sized pair of eyeglasses? Well, wonder no more:

It's called PD, or Pupil Distance. You can measure it with the assistance of a friend, but this is one case where we don't recommend those laser tape measures.