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In the The Not So Short Introduction to LATEX2e - Version 5.01, April 06, 2011 it is suggested that:

A backslash in front of a space generates a space that will not be enlarged. (page 39)

It turns out that this is not really the case. I've been told that \  differs from the normal space token in that it resets the \spacefactor to 1000 (see comment here).

So what does \  actually do? I am not very familiar with the way TeX works and don't really understand what \spacefactor is really responsible for, either. (I looked up the TeXbook, but still don't have any clue.) Can anybody enlighten me?

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1 Answers1

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The "enlarging" the "Not So Short Introduction" refers to has precisely to do with the "space factor".

When the space factor is bigger than 1000 and a space appears, it is enlarged by a quantity that depends on a few quantities (the details are in the TeXbook and in TeX by Topic). The space factor is responsible for the increased space after a period, for instance, when \frenchspacing is not in force.

Each time you use \ , you produce a space that is not influenced by the space factor. More precisely

\  appends glue to the current list, using the same amount that a ⟨space token⟩ inserts in horizontal mode when the space factor is 1000.

It's not correct to say that it sets the space factor to 1000. Indeed \  doesn't change it. Thus

Here ends a sentence.\ And here a new one starts

will not show an enlarged space. The spaces on the line may stretch or shrink to justify the text, but the one after the period will be the same as the others on the same line.

The LaTeX manual proposes a different way to cope with this, namely \@.

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