\sqrtsign and \sqrt

Question

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}.

Answer 1

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).