\addtolength (LaTeX2e unofficial reference manual (November 2018))

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### 14.3 `\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`’.

Unofficial LaTeX2e reference manual