person's acuity is "6/12" ("20/40"), meaning that this person needs to approach to a distance of 6 m to read letters that a person with normal acuity could read at 12 metres (40 ft). In an even more approximate manner, this person could be said to have "half" the normal acuity of 6/6.

At exactly 6 metres' distance from the patient, the letters on the 6/6 line shall subtend 5 minutes of arc (such that the individual limbs of the letters subtend 1 minute of arc), which means that the chart should be sized such that these letters are 8.73 mm