Does a space belong after pt?

Question

Does a space always belong after pt, e.g. as used in \hskip1pt? If a space is used, are there situations where a space will appear where it is not intended? If no space is used, will pt be unrecognized, e.g., if a letter immediately follows, as in 1ptThe?

Answer 1

By the syntax rules of TeX, any length specification allows an optional space after it which is ignored.

The space is always looked for performing macro expansion, which ends when a space or another unexpandable token is found. A space token is then ignored.

So \hskip1pt Then and \hskip1ptThen are perfectly equivalent (but the latter requires a tiny bit of machine work more than the former, because the token T must be put back in the input stream).

However, you should remember that a <skip> specification doesn't end with the 1pt tokens: TeX looks also for a possible plus keyword (or minus, which is looked for after plus <dimen> anyway. So

\hskip1pt plus two

is not equivalent to

\hskip1pt\relax plus two

The former will result in a Missing number error.

So it's best to end incomplete <skip> specifications with \relax, particularly when they are in a macro that might be expanded when there's no full control of the following tokens (see exercise 27.4 in the TeXbook).

Answer 2

The following should answer your question:

\documentclass{article}
\setlength{\parindent}{0pt}% Just for this example
\begin{document}
aaa\hskip20ptbbb \par
aaa\hskip20pt bbb \par
aaa\hskip 20ptbbb \par
aaa\hskip 20pt bbb \par
aaa\rule{20pt}{.4pt}bbb
\end{document}​

It boils down to readability of your code. In that sense, separating lengths from other document elements ("option" 2 & 4 above) makes sense.

Of course, if your editor has syntax highlighting, then perhaps you can live with all possibilities.