Setting bold Greek letters in LaTeX

The simple solution (\mathbf) doesn't always work, because lower-case Greek letters behave differently from upper-case Greek letters (due to Knuth's esoteric font encoding decisions). However, \mathbf can be used for upper-case Greek letters in ordinary circumstances, but the AMS-LaTeX package amsmath.sty 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 package bm.sty, which is part of the LaTeX tools distribution (macros/latex/packages/tools), defines a command \bm which may be used anywhere in maths mode.
• The package amsbsy.sty, which is part of AMS-LaTeX (macros/latex/packages/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.