\thinspace & \negthinspace

Synopsis, one of:

\thinspace
\negthinspace

These produce unbreakable and unstretchable spaces of 1/6em and -1/6em, respectively. These are the text mode equivalents of \, and \! (see Spacing in math mode/\thinspace).

You can use \, as a synonym for \thinspace in text mode.

One common use of \thinspace is as the space between nested quotes:

Killick replied, ``I heard the Captain say, `Ahoy there.'\thinspace''

Another use is that some style guides call for a \thinspace between an ellipsis and a sentence ending period (other style guides, think the three dots and/or four dots are plenty). Another style-specific use is between initials, as in D.\thinspace E.\ Knuth.

LaTeX provides a variety of similar spacing commands (see Spacing in math mode). With amsmath, or as of the 2020-10-01 LaTeX release, they can be used in text mode as well as math mode, including \! for \negthinspace; before that, they were available only in math mode.