Reading charts are often used to measure visual acuity.
Visual acuity measurements evaluate how clearly each eye is seeing. As part of the testing, you are asked to read letters on distance and near reading charts. The results of visual acuity testing are written as a fraction, such as 20/40.

When testing distance vision, the top number in the fraction is the standard distance at which testing is done, twenty feet. The bottom number is the smallest letter size you were able to read. A person with 20/40 visual acuity would have to get within 20 feet of a letter that should be seen at 40 feet in order to see it clearly. Normal distance visual acuity is 20/20.