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, 2.845pt

`cm`

¶-
Centimeter, 10mm

`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.