# What is the difference between ‘truemm’ and ‘mm’ etc.? [duplicate]

Sometimes I see the use of truemm (or true mm) instead of mm (same for other units). So I’m wondering what is the difference between these? Or: Why are some units false?

## marked as duplicate by egreg, Community♦Jul 15 '16 at 9:38

• Where did you see it? In LaTeX or plain TeX files? – egreg Jul 15 '16 at 8:58
• @egreg: Hm … good question … I have no specific file in mind. But the given answers already answer my question :-) – Tobi Jul 15 '16 at 9:18
• I tried true [dimensions], and that tag was missing in the question … I added it now – Tobi Jul 15 '16 at 10:56

No one says it better than Knuth (TeXbook, Chapter 10: Dimensions, page 59).

When you say \magnification=2000, an operation like \vskip.5cm will actually skip 1.0 cm of space in the final document. If you want to specify a dimension in terms of the final size, TeX allows you to say true just before pt, pc, in, bp, cm, mm, dd, cc, and sp. This unmagnifies the units, so that the subsequent magnification will cancel out. For example, \vskip.5truecm is equivalent to \vskip.25cm if you have previously said \magnification=2000. Plain TeX uses this feature in the \magnification command itself: Appendix B includes the instruction

\hsize = 6.5 true in


just after a new magnification has taken effect. This adjusts the line width so that the material on each page will be 6½ inches wide when it is finally printed, regardless of the magnification factor. There will be an inch of margin at both left and right, assuming that the paper is 8½ inches wide.

• Thanks … I guess it’s time to finally buy and read the book myself … – Tobi Jul 15 '16 at 9:18
• @Tobi If you don't the additional markup, you can read the TeXbook from source. I linked it in the question. – Henri Menke Jul 15 '16 at 9:58
• Jep, I know that the source I public but I like printed books :-) – Tobi Jul 15 '16 at 10:55

You can set a magnification with \mag, it is not often used in latex packages but plain has a \magnification macro build on it.

\documentclass{article}
\mag 2000
\begin{document}
\rule{10truemm}{10truemm}
\rule{10mm}{10mm}

\end{document}


• Thanks for the example code! I accepted Henri’s answer since it has more explanations (and he has less reputation ;-)) – Tobi Jul 15 '16 at 9:19
• @Tobi Having less rep shouldn't be the reason for accepting my answer. You should accept the best one! – Henri Menke Jul 15 '16 at 9:59
• @HenriMenke: I know ;-) – Tobi Jul 15 '16 at 10:55