Definition of Twenty-twenty

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Twenty-twenty: Typically considered normal vision. The ability to identify a symbol that subtends an angle of 5 minutes of arc, and score 20/20 on the standard Snellen eye chart named after its inventor. It is usually read while standing a distance of 20 feet from it. Visual acuity is traditionally represented as a fraction, with the distance at which the patient is standing being the numerator, and the normal maximum legible viewing distance ("distance" on the chart) being the denominator. So if, at 20 feet, the patient can read the letters on the row marked "20", the patient has visual acuity of 20/20.