[Tex/LaTex] Usage of LaTeX macro \space (compared with \␣)

spacing

Occasionally I find that internal package code contains the macro \space (defined as \def\space{ } by LaTeX). The most-taught way to produce "explicit" spaces in paragraph text is to use \␣, and I think I haven't seen any source teach \space as a user-level command.

What community knowledge or practices are there regarding the use of \space, especially as compared with \␣?

Here some examples, from the memoir class, which uses \space as well as \␣ in its definitions.

For example it defines \newcommand*{\booknamenum}{\space}; this macro is "called between printing the book name and the number".
A macro the user is more likely to encounter is \printchapternum; memoir defines it using \renewcommand*{\printchapternum}{\centering\chapnumfont \thechapter\space\space}, with two consecutive \spaces.
The class defines \wrappingoff (for use within verbatim environments) via \newcommand*{\wrappingoff}{\def\@xobeysp{\leavevmode\penalty\@M\ }\def\wrapright{}}.

Exhaustive list of use cases for the interword space \␣

(An interesting observation regarding visual appearance within code: a\space\space␣b and a\␣\␣b within paragraph text give seemingly the same output (but note that I haven't tested the exact linebreaking behavior). I personally find \space visually clearer than \␣: when I see the letter string "space", I immediately know what it is. Backslashes and spaces are so ubiquitous in LaTeX that the combination \␣ is harder to make out on the screen. That said, such aspects should only be considered in contexts where there is functionally no difference between the two: only then can it be considered a question of coding style.)

Best Answer

`\space`

\space expands to the normal space token, a character with code of the space (32, 0x20) and the catcode of 10 (space).

In opposite of the space token TeX does not collapse several \space commands and it is not ignored after command names, when reading the input. Therefore it is often used in error messages, warnings and other messages. Example of the LaTeX kernel (latex.ltx):
```
\gdef\@badlinearg{%
  \@latex@error{%
    Bad \protect\line\space or \protect\vector
   \space argument}\@ehb}
```

`\␣`

From "The TeXbook" by Donald E. Knuth, "Chapter 25: Summary of Horizontal Mode":

\␣. A control-space command appends glue to the current list, using the same amount that a ⟨space token⟩ inserts when the space factor is 1000.

As use case it can be used after commands names for typesetting:
```
 \TeX\ is a fascinating language.
```
It can not be used in messages, because it is not expandable. The backslash would also appear in the error message above, if \␣ instead of \space had been used there.
\␣ sets the normal inter-word space (space factor 1000), not the prolonged space after sentences. Thus it can also be used after abbreviations. Example:
```
abc.\ is short for abcdef.\@ and
abcdef.\@ is short for abcdefghi.
```
Here \␣ is shorter to type (2 characters) than \@␣ (3 characters). (\␣ - like \relax and many other primitives - is independent from the space factor: it neither uses nor changes it, but ignores it and uses 1000 instead. Note that \@␣ resets the space factor.) \space cannot be used, because it sets the prolonged space according to the space factor settings.

Related Solutions

[Tex/LaTex] Exhaustive list of use cases for the interword space \␣

The following two constructions,

This is the 1.{} sentence.

and

This is the 1.\ sentence.

have different results. You get a sentential space in the first case and the interword space in the second. Why? In the first case {} begins and ends a group, but in the end, the effect is null: the last item in the horizontal box is a . and spacefactor still has the sentential-space value (3000 by default), so the following space in appended with the sentential spacefactor value. In the second case, \␣, a primitive TeX command, directly appends glue to the horizontal box, "using the same amount that a space token inserts when the space factor is 1000" (TeXbook p. 285).

To extend the example to \␣ vs. {} to usage after an argumentless command, imagine a macro that might expand to something ending with ..

\def\macro{1.}

Then, for the same reason as above, the effect of the following two chunks of code is different.

This is the \macro{} sentence.

This is the \macro\ sentence.

In this case, I would use the second construction, because the assumption is that the result of \macro is a non-sentential phrase, like a single word. If, however, \macro might expand to something sentential, using the first construction would be better since then the ending punctuation of the macro (if any) would control the width of the space following it.

EDIT: As Bruno Le Floch pointed out in a comment, a well-written macro should be defined as \def\macro{1.\@}, "to avoid hiding a change in space factor inside a macro". Thus, in a perfect world, the difference between \macro{} and \macro\␣ disappears.

Regarding \␣ vs. \@␣, I know of no difference between them (barring the pathological situations like some macro tearing apart \@ and ␣ or somebody changing the value of \@m which \@ depends on). Thus I'd agree with the comments that using one or another is truly a matter of style. (By the way, I find it somewhat funny that \␣, which seems more complex on the surface, is actually a primitive TeX command.)

[Tex/LaTex] How to one produce spaces of arbitrary (fixed or relative) width that (do or don’t) linebreak

Maybe this info helps: Lengths and when to use them. You can use different units of measure in the <skip>, for example: pt, ex, em, in, mm or cm among others. It is also valid to use only the <natural width>, for example \hspace{1in}.