Question on where `plus` primitive is defined

Question

I often see either \@plus and plus when specifying the stretchiness of a glue (skip). I can see that \@plus is the same command as plus from latex.ltx (the line \def\@plus{plus}).

But I don't see any plus identified as a tex primitive here nor do I see it described in plain.tex. So what it is? Where is it coming from?

Related: What is a primitive, a macro, and how do they compare? — Werner, Nov 27 '21 at 01:10
By the way, for these things you can lookup by looking up the documentation of the corresponding macro (for example hskip that comes before the thing, then note that it takes a <glue> as argument, then after that lookup what glue means. — user202729, Nov 27 '21 at 05:00

score 5 · Accepted Answer · answered Nov 27 '21 at 01:54

plus is not a primitive itself it (along with minus) is part of the syntax of a glue (skip) length. Similar to the usage of pt or cm which similarly are keywords used in length syntax.

The TeXBook gives the list of such keywords in classical, unextended TeX as

Here is a complete list of TeX's keywords, in case you are wondering about the full set: at, bp, by, cc, cm, dd, depth, em, ex, fil, height, in, l, minus, mm, mu, pc, plus, pt, scaled, sp, spread, to, true, width. (See Appendix~I for references to the contexts in which each of these is recognized as a keyword.)

So it is defined in the web sources of tex-the-program, specifically the code that scans a glue specification:

@ @<Create a new glue specification whose width is |cur_val|...@>=
q:=new_spec(zero_glue); width(q):=cur_val;
if scan_keyword("plus") then
@.plus@>
  begin scan_dimen(mu,true,false);
  stretch(q):=cur_val; stretch_order(q):=cur_order;
  end;
if scan_keyword("minus") then
@.minus@>
  begin scan_dimen(mu,true,false);
  shrink(q):=cur_val; shrink_order(q):=cur_order;
  end;
cur_val:=q

Question on where `plus` primitive is defined

1 Answers1