#### 16.2.2.2 OpenType bold math

Unfortunately, when using the Unicode engines (XeLaTeX, LuaLaTeX), neither `\boldmath` nor `bm` usually work well, because the OpenType math fonts normally used with those engines rarely come with a bold companion, and both `\boldmath` and `bm` require this. (The implementation of `bm` relies on `\boldmath`, so the requirements are the same.) If you do have a bold math font, though, then `\boldmath` and `bm` work fine.

If no such font is available, one alternative is to construct fake bold fonts with the `fontspec` package’s `FakeBold=1` parameter (see its documentation, https://ctan.org/pkg/fontspec). This may be acceptable for drafting or informal distribution, but the results are far from a true bold font.

Another alternative to handling bold for OpenType math fonts is to use the `\symbf` (bold), `\symbfit` (bold italic), and related commands from the `unicode-math` package. These do not change the current font, but rather change the (Unicode) “alphabet” used, which in practice is more widely supported than a separate bold font. Many variations are possible, and so there are subtleties to getting the desired output. As usual, see the package documentation (https://ctan.org/pkg/unicode-math).