\
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 amsmath loaded,$... \mbox{\boldmath$\theta$} ...$
does the trick (and is less bad in regard to superscripts, etc), but is an unsatisfactory solution, too. These problems may be addressed by using a bold mathematics package.$... \text{\boldmath$\theta$} ...$
\
bm
which may be used anywhere
in maths mode.
\
boldsymbol
, which (though slightly less
comprehensive than \
bm
) covers almost all common cases.
This answer last edited: 2013-06-12
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=boldgreek