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14.3 `\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?
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’.