A common requirement ill-defined.
From time to time I come across some document that is specified to be typeset using “double-spacing”. But in my experience, that term is not well defined.
Typefaces are often specified in terms of point72 point = 1 inch; 3600 point = 127 cm size. Ostensibly, this is the distance from the lowest ascender to the highest ascender in the typeface, though things like accents on capitals (Å) can violate that and some digital fonts ignore this correlation altogether, picking glyph size and font size apparently at random.
Type is placed on the page with some leading“leading”’s pronounced like “led”, referring to the strips of lead used to keep lines of movable type apart. so that adjacent lines of text do not touch. The leading is the gap between lines, while the line height is the distance between the baseline of adjacent lines of text. Thus, a 12-point font with 3 points of leading has a 15-point line height. This is sometimes written “12/15”.
The notion of single- and double-spacing are not well defined typographically. Single-spacing is usually understood to be the “normal” amount of leading, often 15-20% of the size of the font used. On a typewriter, double spacing means “every other line is blank”, so if normal text were 12/15 then a typewriter double-spaced text would be 12/30, with 18 points of leading instead of 3. But double-spacing is also sometimes used to mean 100% leading, so 12/15 would be double-spaced as 12/24 instead. There are even some that go for a more visual doublespacing, realizing that most text only fills the area between the baseline and the median and that there is no need to add in both the ascender and descender heights to look double-spaced so 12/15 might be double-spaced as 12/21 or something near that.
I never know how to react when some document is requested to be “double-spaced”. Sometimes I wish someone had been at the reigns when so many crafts entered the public domain through the digital age.