emacs internals trivia

[Scott Lanning]

There are several macros in lisp.h (section /* Data type checking */)
that are like CHECK_foo(x, i), but it seems that the second argument
is useless! I thought maybe it's for backward compatibility, like
the `i' used to be used for something, then something changed and
it was useless but people didn't want to change all those old calls
to CHECK_STRING, and so on. I checked emacs-18.59, but it's not
even there. So the question: am I right about the backward
compatibility, or did I overlook something?

Also, what does 'Q' stand for in Qnil (et al)?
My observation is that 'V' means a customizable variable
while 'Q' means constant or something that is "interned" in emacs.
Also I guess it's related to Lisp convention, but I can't figure
what 'Q' stands for (like maybe it's a Latin word).