The Snellen Eye test uses the idea of a subtended 5 minutes of arc with a 20 foot distance. This became the "standard vision" measurement. Also, Snellen developed the idea of a 5 x 5 grid for a standard for his optotypes. Each optotype must meet certain percentage requirements in this grid before it is used for testing. This helps to make a uniform and equal vision chart. The Snellen eye chart is the basis for many other vision tests.