Sometimes I see the use of
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?
No one says it better than Knuth (TeXbook, Chapter 10: Dimensions, page 59).
When you say
\magnification=2000, an operation like
\vskip.5cmwill 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
sp. This unmagnifies the units, so that the subsequent magnification will cancel out. For example,
\vskip.5truecmis equivalent to
\vskip.25cmif you have previously said
\magnification=2000. Plain TeX uses this feature in the
\magnificationcommand 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.