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I know there are TeX primitives and Plain TeX macros created (directly/indirectly) by TeX primitives. My question is are there TeX macro? E. g. I can use \break in pdfTeX without defining it and \break is not a TeX primitive. Is it a TeX macro or a Plain TeX macro? Plain TeX is a format like LaTeX. Does the TeX engine load Plain TeX by default? If not \break must be a TeX macro.

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  • There are primitives, and there are macros: how we 'call' the latter is something of a semantic issue. For example, LaTeX defines \centerline as in plain TeX, but it's not a documented part of LaTeX ...
    – Joseph Wright ♦
    Oct 27, 2018 at 18:57
  • Does the TeX engine load Plain TeX by default? — yes if you invoke it as tex or pdftex (in a typical TeX distribution), it does load plain TeX by default (you should see something like “preloaded format=pdftex” or “preloaded format=tex” in the first line of the program output, which are both references to plain).
    – ShreevatsaR
    Oct 27, 2018 at 19:29

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When TeX starts, it does not know a single macro, only primitives. If it runs as iniTeX, it expects to be given a file to execute (or you could enter commands interactively), and that file could (and for sure, will) define a bunch of macros. If it does not run as iniTeX, it expects to be told the name of a format file, which will also contain a bunch of macro definitions, among other things. (Normally, TeX looks at the name it is being called by to figure out what format to load.) By the way, a format file is nothing but a binary representation of the internal state of iniTeX before it executed a \dump primitive.

In particular, no, TeX does not load the plain macros by default, and no, there are no “TeX macros” the way you think there are, if I understood you correctly.

However, the LaTeX format does contain the vast majority of plain TeX macros, sometimes unchanged, sometimes in modified form.

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    When I execute pdfTeX (miktex-pdftex.exe) I can use \break macro and as the TeXbook say \break is a plain TeX macro. How does miktex-pdftex.exe know to load Plain TeX? Why are Plain TeX macro available without explicitly being loaded? The only argument of miktex-pdftex.exe is `$fullname``.
    – John webner
    Oct 27, 2018 at 19:33
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    +1, but "TeX does not load the plain macros by default" might be correct in theory, although in practice the user will call something like "pdftex" which preloads plain.
    – TeXnician
    Oct 27, 2018 at 19:33
  • @Johnwebner Simply start pdftex without any argument, you will drop into interactive mode and have something like preloaded format=pdftex in the first few lines.
    – TeXnician
    Oct 27, 2018 at 19:34
  • As I said in my answer, if it does not run as iniTeX, it will typically look at the name it is being run as (for example, pdftex or miktex-pdftex.exe, apparently), and use that information to decide what preloaded format to load. You can override that with the command line option -fmt=…, naming the format to be loaded.
    – Harald Hanche-Olsen
    Oct 27, 2018 at 21:06
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    The point made by @TeXnician (and I agree) is that statements like “When TeX starts, it does not know a single macro…” and “TeX does not load the plain macro by default” are rather misleading, because “by default” one invokes TeX as tex or pdftex or something like that; it's the -ini form (INITEX) that is rare and unusual (i.e., not the default) — calling INITEX the default is a very unconventional usage of “default” IMO; even Knuth's original design had “TEX” like present-day tex (called the “production version of TeX” in the TeX program source code), and “INITEX” was special.
    – ShreevatsaR
    Oct 27, 2018 at 21:23

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