The Snellen eye chart is a diagram of different-sized letters that you see at your eye doctor’s office. Interestingly, it’s been around for over 150 years and helps determine the strength of your eyesight. The chart also indicates if you meet the legal vision requirement to obtain a driver’s license. Here’s a look at the history of the Snellen Eye Chart, how it’s used, and what it means to have 20/20 vision.