Letter size
Snellen used a rather verbose definition to indicate the letter size: “the distance in meters (feet) at which the letter subtends 5 min of arc.” Louise Sloan simplified this by defining the M-unit as the size that subtends 5 min of arc at 1 meter. Thus, the verbose statement “this letter subtends 5 min of arc at ‘x’ meter” can be simplified to “this letter is ‘x’ M-units”. Here the number denoting the M unit size is the viewing distance in meters for 20/20 acuity.