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\addtolength

Synopsis:

\addtolength{len}{amount}

Increment the length len by amount. The length name len begins with a backslash, \. 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 the length len with \newlength, 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’.