When you visit the eye doctor for a checkup, you may be asked to read an eye chart. The chart measures your visual acuity, or sharpness of vision. If you don’t wear glasses or contacts, your eye doctor will use the results to find out whether you need them. If you already wear corrective lenses, the test results will tell your doctor if your glasses or contacts prescription needs to change.

The most commonly used eye chart is known as the Snellen chart. It usually shows 11 rows of capital letters. The first line has one very large letter. Each row after that has increasing numbers of letters that are smaller in size.

You stand 20 feet away from the chart, and read from it without your glasses or contacts. You cover one eye and read out the smallest line of letters you can see. The test is done on each eye. In some offices, the chart is viewed using a mirror so the test can be done with less than 20 feet of space. The results are the same whether or not a mirror is used.

If you have 20/20 vision, you are considered to have normal visual acuity.

The top number refers to the distance in feet that you stand from the chart. The bottom number indicates the distance at which a person with normal eyesight can read the same line you correctly read. A person with 20/20 vision can see what an average person can see on an eye chart when they are standing 20 feet away.