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The following lines caught my attention while browsing source2e while looking for the definition of another command.
\DeclareMathRadical{\sqrtsign}{symbols}{"70}{largesymbols}{"70}
and
\DeclareRobustCommand\sqrt{\@ifnextchar[\@sqrt\sqrtsign}
\def\@sqrt[#1]{\root #1\of}
What was \sqrtsign for? I tried \sqrtsign{x} and it outputs something similar to \sqrt{x}.
The definition of LaTeX's \sqrt is
\DeclareRobustCommand\sqrt{\@ifnextchar[\@sqrt\sqrtsign}
\def\@sqrt[#1]{\root #1\of}
which means that if the optional argument is used (i.e., if the next character after \sqrt is [), \@sqrt is used (and this in its turn, uses \root#1\of
whose definition can be found on page 210 of source2e.pdf) to typeset the radical index and the subradical expression; if no optional argument is given,\sqrtsign is used to typeset the subradical expression (\sqrtsign doesn't allow an optional argument).
