gtroff (like many other programs) requires numeric parameters to
specify various measurements. Most numeric parameters9 may have a
measurement unit attached. These units are specified as a single
character that immediately follows the number or expression. Each of
these units are understood, by
gtroff, to be a multiple of its
basic unit. So, whenever a different measurement unit is
gtroff converts this into its basic units. This
basic unit, represented by a ‘u’, is a device dependent measurement,
which is quite small, ranging from 1/75th to 1/72000th of an
inch. The values may be given as fractional numbers; however,
fractional basic units are always rounded to integers.
Some of the measurement units are completely independent of any of the
current settings (e.g. type size) of
Although groff’s basic unit is device-dependent, it may still be smaller
than the smallest unit the device is capable of producing. The register
.H specifies how many groff basic units constitute the current
device’s basic unit horizontally, and the register
this value vertically.
Inches. An antiquated measurement unit still in use in certain backwards countries with incredibly low-cost computer equipment. One inch is defined to be 2.54 cm (worldwide since 1964).
Centimeters. One centimeter is about 0.3937 in.
Points. This is a typesetter’s measurement used for measure type size. It is 72 points to an inch.
Pica. Another typesetting measurement. 6 picas to an inch (and 12 points to a pica).
See Fractional Type Sizes, for a discussion of these units.
Fractions. Value is 65536. See Colors, for usage.
The other measurements understood by
gtroff depend on settings
currently in effect in
gtroff. These are very useful for
specifying measurements that should look proper with any size of text.
Ems. This unit is equal to the current font size in points. So called because it is approximately the width of the letter ‘m’ in the current font.
groff, this is half of an em.
Vertical space. This is equivalent to the current line spacing. See Sizes, for more information about this.
100ths of an em.
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