\addtolength ¶Synopsis:
\addtolength{\len}{amount}
Increment the length \len by amount. 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. The amount is a rubber length
(see Lengths). It can be positive, negative or zero, and can be in
any units that LaTeX understands (see Units of length).
Below, if \parskip starts with the value 0pt plus 1pt
Doctor: how is the boy who swallowed the silver dollar?
\addtolength{\parskip}{1pt}
Nurse: no change.
then it has the value 1pt plus 1pt for the second paragraph.
If you did not declare \len with \newlength, for example if
you mistype it as in
\newlength{\specparindent}\addtolength{\sepcparindent}{...},
then you get an error like ‘Undefined control sequence. <argument>
\sepcindent’. If the amount uses some length that has not been
declared, for instance if for example you mistype the above as
\addtolength{\specparindent}{0.6\praindent}, then you get
something like ‘Undefined control sequence. <argument> \praindent’.
If you leave off the backslash at the start of \len, as in
\addtolength{parindent}{1pt}, then you get something like
‘You can't use `the letter p' after \advance’.