For Example, If \thinmskip = 3mu, This Makes \thickmskip = 6mu.

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For example, if \thinmskip = 3mu, this makes \thickmskip = 6mu. But if
you also want to use \skip12 for horizontal glue, whether in math mode or
not, the amount of skipping will be in points (e.g., 6pt). The rule is
that glue in math mode varies with the size only when it is an \mskip;
when moving between an mskipand ordinary skip, the conversion factor
1mu=1pt is always used. The meaning of '\mskip\skip12' and
'\baselineskip=\the\thickmskip' should be clear.
-- Donald Knuth, TeX 82 -- Comparison with TeX80

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