What does \normallineskiplimit do?

Question

In this excellent answer, one of the tools used is to redefine \normallineskiplimit. I cannot find this macro in any reference, although it does show up in this answer. Where is this length originally defined, and what exactly does this macro control?

A good answer may need to address related macros (if they exist) such as \normallineskip.

Answer 1

\normallineskiplimit (and friends) are used in two places in latex,

they are used in \@arrayparboxrestore

\lineskip\normallineskip
\baselineskip\normalbaselineskip

so that inside \parbox and minipage and tabular p columns the baselineskip is restored to a normal state even if used in a context where they ave non standard values.

\displaylines compensates for non standard values of \lineskiplimit

\def\displ@y{\global\dt@ptrue\openup\jot\m@th
  \everycr{\noalign{\ifdt@p \global\dt@pfalse \ifdim\prevdepth>-1000\p@
      \vskip-\lineskiplimit \vskip\normallineskiplimit \fi
      \else \penalty\interdisplaylinepenalty \fi}}}

but that is really a hangover from plain TeX and not used much in LaTeX.

However the AMS alignments all reset this in a similar way amsmath has

\def\@display@init#1{%
    \global\dt@ptrue \spread@equation
    \everycr{%
        \noalign{%
            #1%
            \ifdt@p
                \global\dt@pfalse
                \vskip-\lineskiplimit
                \vskip\normallineskiplimit
            \else
                \penalty\@eqpen \global\dspbrk@lvl\m@ne
            \fi
        }%
    }%
}

to adjust for the difference between the current value of \lineskiplimit and the saved \normallineskiplimit

You can also use \normalbaselines to explicitly reset the values to defaults

\def\normalbaselines{\lineskip\normallineskip
  \baselineskip\normalbaselineskip \lineskiplimit\normallineskiplimit}

but I'm pretty sure I've never needed to do that:-)