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).

In the default configuration, lower-case Greek letters behave differently from upper-case Greek letters (the lower-case greek letters are in the maths fonts, while the upper-case letters are in the original (OT1-encoded) text fonts).

The Plain TeX solution *does* work, in a limited way; you set a
maths *style*, before you start an equation; thus

does the job, but{\boldmath$\theta$}

`\boldmath`

may not be used in maths mode. As a
result, this solution requires that you embed single bold characters
in a text box:
which then causes problems in superscripts, etc. With$... \mbox{\boldmath$\theta$} ...$

does the trick (and is less bad in regard to superscripts, etc), but is an unsatisfactory solution, too.$... \text{\boldmath$\theta$} ...$

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 apply to all mathematical symbols, not merely Greek letters.

- bm.sty
- Distributed as part of 2etools
- amsbsy.sty
- Distributed as part of AMSLaTeX amslatex
- amsmath.sty
- Distributed as part of AMSLaTeX amslatex

This answer last edited: 2013-06-12

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