Outside of the US, the standard chart distance is 6 m, normal acuity is designated 6/6, and other acuities are expressed as ratios with a numerator of 6. Many rooms do not have 6 m available, and either a half-size chart subtending the same angles at 3 m, or a reversed chart projected and viewed by a mirror is used.
Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription. During such examinations, acuity ratios are never mentioned.
The largest letter on an eye chart often represents an acuity of 6/60 (20/200), the value that is considered "legally blind." Some individuals with moderate myopia may not be able to read the large E without glasses, but have no problem reading the 6/6.0 (20/20) line or 6/4.5 (20/15) line with glasses. By contrast, legally blind individuals have a visual acuity of 6/60 (20/200) or less when using the best corrective lens.