$ \mathbf{xy} A $
You may well find that this behaves as if you had put:
$ \bf {xy} A $
everything including the A coming out bold.
LATEX 2.09 allowed sites to customize their LATEX installation,
which resulted in documents producing different results on different
LATEX installations. LATEX2e no longer allows so much
customization but, for compatibility with old documents, the local
configuration file latex209.cfg is loaded every time LATEX2e
enters LATEX 2.09 compatibility mode.
For example, if your site was customized to use the New Font
Selection Scheme (NFSS) with the oldlfont option, then you can
make LATEX2e emulate this by creating a latex209.cfg file
containing the commands:
\ExecuteOptions{oldlfont}\RequirePackage{oldlfont}
Similarly, to emulate NFSS with the newlfont option, you can
create a latex209.cfg file containing:
\ExecuteOptions{newlfont}\RequirePackage{newlfont}