UK TeX FAQ -- question 209

### Setting bold Greek letters in LaTeX

The issue here is complicated by the fact that `\mathbf` (the command for setting bold text in TeX maths) affects a select few mathematical symbols (the uppercase Greek letters). However lower-case Greek letters behave differently from upper-case Greek letters (due to Knuth's esoteric font encoding decisions). However, `\mathbf` can't be used even for upper-case Greek letters in the AMSLaTeX amsmath package, which disables this font-switching and you must use one of the techniques outlined below.

The Plain TeX solution does work, in a limited way:

      {\boldmath$\theta$}

but `\boldmath` may not be used in maths mode, so this 'solution' requires arcana such as:

      $... \mbox{\boldmath$\theta$} ...$

which then causes problems in superscripts, etc.

These problems may be addressed by using a bold mathematics package.

- The bm package, which is part of the LaTeX tools distribution, defines a command `\bm` which may be used anywhere in maths mode.
- The amsbsy package (which is part of AMSLaTeX) defines a command `\boldsymbol`, which (though slightly less comprehensive than `\bm`) covers almost all common cases.

All these solutions cover all mathematical symbols, not merely Greek letters.

`bm.sty`
  Distributed as part of macros/latex/required/tools.tar.gz
`amsbsy.sty`
  Distributed as part of AMSLaTeX macros/latex/required/amslatex.tar.gz
`amsmath.sty`
  Distributed as part of AMSLaTeX macros/latex/required/amslatex.tar.gz