5.18 Sizes

gtroff uses two dimensions with each line of text, type size and vertical spacing. The type size is approximately the height of the tallest glyph.²¹ Vertical spacing is the amount of space gtroff allows for a line of text; normally, this is about 20% larger than the current type size. Ratios smaller than this can result in hard-to-read text; larger than this, it spreads the text out more vertically (useful for term papers). By default, gtroff uses 10 point type on 12 point spacing.

The difference between type size and vertical spacing is known, by typesetters, as leading (this is pronounced ‘ledding’).