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In the documentation for the ifthen package, there is a test called \lengthtest which compares two <dimen> parameters with the operators <, > or =. (For other test commands, <dimen> is usually <string> or <number>.) Searching for "dimen" in the document and on google, I find nothing to explain what it means.

What is <dimen>?

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up vote 10 down vote accepted

AFAIK, this would be referring to a length, so, either 1pt, 2cm, 5mm, 10em, etc. or previously defined lengths: \textwidth, \parskip, 4\itemsep, etc.

This would come in handy, for example, if you want to do one thing if the length of your title is larger than the width of the page, and a different thing if it is smaller than it...

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Makes sense, now that you say it... – Tomas Lycken Nov 2 '10 at 23:21
DEK originally used <dimen>; Leslie Lamport changed the name to <length>. – Matthew Leingang Nov 3 '10 at 9:44
\parskip and \itemsep are both skips, not dimensions. – TH. May 15 '11 at 0:51

Quoting from Donald E. Knuth, The TeXbook, pages 59, 61:

Henceforth we shall use the term <dimen> to stand for a legitimate TeX dimension. [...] [A] complete definition of everything that a <dimen> can be will be given in Chapter 24.

My (hopefully educated) guess is that this is what the ifthen documentation refers to.

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