# [texhax] Constructing a very thin space

Paul Isambert zappathustra at free.fr
Fri Jul 26 11:32:12 CEST 2013

Philip Taylor <P.Taylor at Rhul.Ac.Uk> a écrit:
> Karl Berry wrote:
>
> > I do not have an example at hand, but without the space after em',
> > TeX will still be reading and expanding tokens after the macro is
> > called (to look for the <space> that's supposed to be there).
>
> To look for the <space> that's supposed to be /where/, Karl ?
>
> Considering \thinspace as an example, and using DEK's
> definition from Plain.TeX :
>
> \def\thinspace{\kern .16667em }
>
> No space is expected after \thinspace, which is automatically
> terminated by any non-letter; no space is expected after \kern .16667em,
> since all 2-letter units are self-terminating and
> there are no optional plus/minus modifiers, so I cannot see
> to what "<space> that's supposed to be there" you are referring.

Units can be followed by an optional space, so they’re not
“self-terminating”; if \thinspace did not include a space in its
definition, expansion would continue until a space is found, see:

\def\aspace{ }
a\kern0pt\aspace b

with no space between the two letters. Similarly, “a\kern0ptb” and
“a\kern0pt b” are the same thing. So the space at the end of
\thinspace is quite important.

Best,
Paul

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