`gtroff`

(like many other programs) requires numeric parameters to
specify various measurements. Most numeric parameters^{1} may have a
measurement unit attached. These units are specified as a single
character which 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
specified `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 `gtroff`

.

`i`

- Inches. An antiquated measurement unit still in use in certain
backwards countries with incredibly low-cost computer equipment. One
inch is equal to 2.54cm.
`c`

- Centimeters. One centimeter is equal to 0.3937in.
`p`

- Points. This is a typesetter's measurement used for measure type size.
It is 72 points to an inch.
`P`

- Pica. Another typesetting measurement. 6 Picas to an inch (and
12 points to a pica).
`s`

`z`

- See Fractional Type Sizes, for a discussion of these units.
`f`

- 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 which should look proper with any size of text.

`m`

- 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. `n`

- Ens. In
`groff`

, this is half of an em. `v`

- Vertical space. This is equivalent to the current line spacing.
See Sizes, for more information about this.
`M`

- 100ths of an em.