# References for \dimexpr \numexpr

I'm not really sure whether this is the right place to ask this question, but here I go....

Over the holiday I've been taking time to read through a lot of the Q&A here. I've been running across some TeX code (or maybe LaTeX code) that I cannot find either in source2e doc or Knuth's The TeXbook. Where can I find documentation that explains `\dimexpr` and `\numexpr` and their syntax?

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These are primitives which are not present in Knuth's TeX but which were added as part of the e-TeX extensions. As such, they are documented in the e-TeX manual, which is most conveniently accessed using `texdoc etex`.