The chart consists of 11 lines of block letters, beginning with a large single letter on the top row. The number of letters on each row increases moving from top to bottom. The size of the letters progressively decreases, allowing for more letters on each subsequent line. The traditional Snellen eye chart only makes use of the following letters: C, D, E, F, L, O, P, T and Z.
When testing for visual acuity, the patient covers one eye and reads aloud the letters on the chart, beginning at the top and moving toward the bottom. The smallest row of letters that the patient reads accurately determines visual acuity in the uncovered eye. The test is repeated with the other eye, and then with both eyes together. Visual acuity is sometimes expressed as 20/20, or a similar number, meaning the smallest letters accurately read on the chart.
There are several versions of this chart for people, such as young children, who cannot read the letters of the alphabet.