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Units of length

TeX and LaTeX know about these units both inside and outside of math mode.

pt ¶

Point, 1/72.27 inch. The (approximate) conversion to metric units is 1point = .35146mm = .035146cm.

pc ¶

Pica, 12 pt

in ¶

Inch, 72.27 pt

bp ¶

Big point, 1/72 inch. This length is the definition of a point in PostScript and many desktop publishing systems.

mm ¶

Millimeter, 2.845 pt

cm ¶

Centimeter, 10 mm

dd ¶

Didot point, 1.07 pt

cc ¶

Cicero, 12 dd

sp ¶

Scaled point, 1/65536 pt

Three other units are defined according to the current font, rather than being an absolute dimension.

ex ¶

The x-height of the current font ex, traditionally the height of the lowercase letter x, is often used for vertical lengths.

em ¶

Similarly em, traditionally the width of the capital letter M, is often used for horizontal lengths. This is also often the size of the current font, e.g., a nominal 10pt font will have 1em = 10pt. LaTeX has several commands to produce horizontal spaces based on the em (see \enspace & \quad & \qquad).

mu ¶

Finally, in math mode, many definitions are expressed in terms of the math unit mu, defined by 1em = 18mu, where the em is taken from the current math symbols family. See Spacing in math mode.

Using these units can help make a definition work better across font changes. For example, a definition of the vertical space between list items given as \setlength{\itemsep}{1ex plus 0.05ex minus 0.01ex} is more likely to still be reasonable if the font is changed than a definition given in points.