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\settodepth

Synopsis:

\settodepth{len}{text}

Set the length len to the depth of box that LaTeX gets on typesetting the text argument. The length name len must begin with a backslash, \.

This will show how low the character descenders go.

\newlength{\alphabetdepth}
\settodepth{\alphabetdepth}{abcdefghijklmnopqrstuvwxyz}
\the\alphabetdepth

If you did not set aside the length len, if for example you mistype the above as \settodepth{\aplhabetdepth}{abc...}, then you get something like ‘Undefined control sequence. <argument> \aplhabetdepth’. If you leave the backslash out of len, as in \settodepth{alphabetdepth}{...} then you get something like ‘Missing number, treated as zero. <to be read again> \setbox’.