Fonts in LaTeX are characterized by four attributes (see fntguide.pdf for more information):
- encoding
- family
- series
- shape
If we look in fontmath.ltx, we see
\DeclareMathAlphabet{\mathbf}{OT1}{cmr}{bx}{n}
where the arguments after the command name are in the same order as outlined above. This is the math analog of asking
\usefont{OT1}{cmr}{bx}{n}
in text, where all four attributes are specified. It's different from \bfseries in that the latter only selects the \bfdefault attributes (usually b or bx).
What family names are legal? You can define yourself a family in your document with
\DeclareFontFamily{<encoding>}{<name>}{<default commands>}
and supplement it with the appropriate \DeclareFontShape declarations to associate physical fonts to this family. If a font family name, say foo, is unknown to LaTeX, a search in the LaTeX trees is performed for files named <enc>foo.fd, where <enc> means the encoding current at the time of the font selection request (in uppercased or lowercased versions). If no such file is found, the family name will be substituted with the default cmr.
Now you know what family names are legal: just browse your TeX distribution for files with extension .fd. And looking in them you can see what series and shape combinations are available.
Caveat. Several .fd files depend on bits of information gathered elsewhere, because they are accompanying specific packages (an outstanding example are the .fd files that accompany newtxtext and newtxmath), so using them may not be possible “directly”, but the relevant information can usually be extracted without too much labor.
