# How to define a math command that displays a variable in a specific font?

How to define a math command that displays a variable in a specific font? E.g. usage \MyMathCommand{P}.

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\newcommand{\MyMathCommand}[1]{\mathbf{#1}}

which would print the variable in boldface (perhaps typical of matrix or vector notation). There is also \mathit (for italic math) and \mathsf (for sans serif fonts). These can be mixed in some fonts. You may be interested also in \mathcal (for calligraphy-style letters, perhaps used in set notation) or something else altogether.
\newcommand{\MyMathCommand}[1]{\ensuremath{\mathbf{#1}}}