[texhax] math \fontdimen

[Karl Berry]

    Now I wonder a little whether I can be sure that (outside PlainTeX) 2 will 
    be the symbols family.

Yes.  It is built into TeX: The Program that family 2 is the symbols
family (and 3 is math extension).  The math typesetting code takes
dimens from those families.

I believe it is not strictly necessary that family 1 be math italic, but
I'm not sure anyone has gone against it in practice.