21) How do I get symbols for ``the real numbers'', ``the complex numbers'',
    and so on?

    These symbols are known as ``blackboard bold'' and are available in
    the AMS fonts ``msam'' (e.g., ``msam10'' for 10pt) and ``msbm''. 
    They replace the older ``msxm'' and ``msym''. The fonts have a large
    number of mathematical symbols to supplement the ones 
    provided by TeX. The fonts are available via anonymous ftp from
    e-math.ams.com (130.44.1.100) in the directory ./ams/amsfonts.
    Two files which load the fonts and define the symbols are provided,
    and both work with either TeX or LaTeX. Questions or suggestions 
    regarding these fonts should be directed to 
    tech-support@math.ams.com.

    A set of LaTeX macros for a ``lazy person's'' blackboard bold are:
       \newcommand{\R}{{\sf R\hspace*{-0.9ex}\rule{0.15ex}%
       {1.5ex}\hspace*{0.9ex}}}
       \newcommand{\N}{{\sf N\hspace*{-1.0ex}\rule{0.15ex}%
       {1.3ex}\hspace*{1.0ex}}}
       \newcommand{\Q}{{\sf Q\hspace*{-1.1ex}\rule{0.15ex}%
       {1.5ex}\hspace*{1.1ex}}}
       \newcommand{\C}{{\sf C\hspace*{-0.9ex}\rule{0.15ex}%
       {1.3ex}\hspace*{0.9ex}}}