2,000 English idioms, phrases and expressions that we use daily, with their meanings and origins explained...
1 An abbreviation might be formed using the first and last letter of the complete word.  2 Yard, for example, is abbreviated yd., and quart is abbreviated qt.. 3  Some measurement abbreviations are formed by using the first letter or two (or sometimes three) of the word. 4  Some examples of this are mi. ( 5 mile), in.
By custom, the ratio is often stated with a standard testing distance such as 20 feet or 6 meters; thus 20/100 (or 6/30) means that the person located 20 feet (or 6 meters, respectively) from the chart can identify a letter subtending an angle of one arcminute at a distance of 100 feet (30 meters).