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.1 Vertical spacing is the amount
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').
 This is usually the parenthesis. Note that in most cases the real dimensions of the glyphs in a font are not related to its type size! For example, the standard PostScript font families `Times Roman', `Helvetica', and `Courier' can't be used together at 10pt; to get acceptable output, the size of `Helvetica' has to be reduced by one point, and the size of `Courier' must be increased by one point.