Units of length

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

pt

Point, 1/72.27 inch. The conversion to metric units is 1point = 2.845mm = .2845cm.

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.

cm

Centimeter

mm

Millimeter

dd

Didot point, 1.07 pt

cc

Cicero, 12 dd

sp

Scaled point, 1/65536 pt

Two other lengths that are often used are values set by the designer of the font. The x-height of the current font ex, traditionally the height of the lowercase letter x, is often used for vertical lengths. Similarly em, traditionally the width of the capital letter M, is often used for horizontal lengths (there is also \enspace, which is 0.5em). Use of these 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.

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