\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.