14.6 \settowidth

Synopsis:

\settowidth{\len}{text}

Set the length \len to the width of the box that LaTeX gets on typesetting the text argument. The length name \len has to be a control sequence (see Control sequence, control word and control symbol), and as such must begin with a backslash, \ under normal circumstances.

This prints the width of the lowercase ASCII alphabet.

\newlength{\alphabetwidth}
\settowidth{\alphabetwidth}{abcdefghijklmnopqrstuvwxyz}
\the\alphabetwidth

If no such length \len has been declared with \newlength, if for example you mistype the above as \settowidth{\aplhabetwidth}{abc...}, then you get something like ‘Undefined control sequence. <argument> \aplhabetwidth’. If you leave the backslash out of \len, as in \settoheight{alphabetwidth}{...} then you get something like ‘Missing number, treated as zero. <to be read again> \setbox’.