[OS X TeX] which is real math mode?

[Matthew Inglis]

> \begin{displaymath} puts one into a displayed math environment,  
> like to
> the $$ 2 + 2 = 4 $$ of Plain TeX (which should not be used in LaTeX).

Why shouldn't $$...$$ be used in LaTeX? I have heard several people  
give this advice, but have always cheerfully ignored it with no  
obvious adverse consequences. Can anyone enlighten?

Matthew.

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