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When typesetting mathematics, LaTeX puts in spacing according to the
normal rules for mathematics texts. If you enter y=m x then
LaTeX ignores the space and in the output the m is next to the x,
as y=mx.
But LaTeX’s rules occasionally need tweaking. For example, in an
integral the tradition is to put a small extra space between the
f(x) and the dx, here done with the \, command:
\int_0^1 f(x)\,dx
LaTeX provides the commands that follow for use in math mode. Many
of these spacing definitions are expressed in terms of the math unit
mu. It is defined as 1/18em, where the em is taken from the
current math symbols family (see Units of length). Thus, a
\thickspace is something like 5/18 times the width of
a ‘M’.
\; ¶Synonym: \thickspace. Normally 5.0mu plus 5.0mu. With
the amsmath package, or as of the 2020-10-01 LaTeX release,
can be used in text mode as well as math mode; otherwise, in math mode
only.
\negthickspace ¶Normally -5.0mu plus 2.0mu minus 4.0mu. With the amsmath
package, or as of the 2020-10-01 LaTeX release, can be used in text
mode as well as math mode; otherwise, in math mode only.
\: ¶\>Synonym: \medspace. Normally 4.0mu plus 2.0mu minus
4.0mu. With the amsmath package, or as of the 2020-10-01
LaTeX release, can be used in text mode as well as math mode; before
that, in math mode only.
\negmedspace ¶Normally -4.0mu plus 2.0mu minus 4.0mu. With the amsmath
package, or as of the 2020-10-01 LaTeX release, can be used in text
mode as well as math mode; before that, in math mode only.
\, ¶Synonym: \thinspace. Normally 3mu, which is 1/6em.
Can be used in both math mode and text mode (see \thinspace & \negthinspace).
This space is widely used, for instance between the function and the
infinitesimal in an integral \int f(x)\,dx and, if an author does
this, before punctuation in a displayed equation.
The antiderivative is
\begin{equation}
3x^{-1/2}+3^{1/2}\,.
\end{equation}
\! ¶Synonym: \negthinspace. A negative thin space. Normally
-3mu. With the amsmath package, or as of the 2020-10-01
LaTeX release, can be used in text mode as well as math mode;
otherwise, the \! command is math mode only but the
\negthinspace command has always also worked in text mode
(see \thinspace & \negthinspace).
\quad ¶This is 18mu, that is, 1em. This is often used for space
surrounding equations or expressions, for instance for the space between
two equations inside a displaymath environment. It is available
in both text and math mode.
\qquad ¶A length of 2 quads, that is, 36mu = 2em. It is available in both text and math mode.