\boldmath & \unboldmath (LaTeX2e unofficial reference manual (November 2018))

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#### 16.2.2 `\boldmath` & `\unboldmath`

Synopsis (used in paragraph mode or LR mode):

    \boldmath \( math \)

or

    \unboldmath \( math \)

Declarations to change the letters and symbols in math to be in a bold font, or to countermand that and bring back the regular (non-bold) default. They must be used when not in math mode or display math mode (see Modes). Both commands are fragile (see \protect).

In this example each `\boldmath` command takes place inside an `\mbox`,

    we have $\mbox{\boldmath \( v \)} = 5\cdot\mbox{\boldmath \( u \)$}$

which means `\boldmath` is only called in a text mode, here LR mode, and explains why LaTeX must switch to math mode to set `v` and `u`.

If you use either command inside math mode, as with `Trouble: \( \boldmath x \)`, then you get something like ‘`LaTeX Font Warning: Command \boldmath invalid in math mode on input line 11`’ and ‘`LaTeX Font Warning: Command \mathversion invalid in math mode on input line 11`’.

There are many issues with `\boldmath`. New documents should use the `bm` package provided by the LaTeX Project team. A complete description is outside the scope of this document (see the full documentation on CTAN) but even this small example

    \usepackage{bm}   % in preamble
    ...
    we have $\bm{v} = 5\cdot\bm{u}$

shows that it is an improvement over `\boldmath`.

Unofficial LaTeX2e reference manual