Having just been annoyed (ok, ok, I'm an old grouch) for the $n$-th time by seeing a question mis-tagged (indeed, mis-titled) propositional-calculus, I've just realized there is no appropriate tag for questions that concern predicate logic, predicate calculus, quantificational logic, first-order logic (whatever you want to call it).
So, hopefully to get the ball rolling, I've retagged the last couple predicate-logic. I think that's probably to be preferred to two of the other options ("calculus" is a bit old-school, "first-order logic" could be a bit restrictive). But maybe others would prefer "quantificational logic"? But we should surely encourage the use of one or the other and deprecate the use of "propositional-logic" for questions that essential concern the logic of quantifiers.
Edit [Jan. $27$th A.K.]:
Can we please make agree on some reasonable state, the current one is heading towards being very messy.
The main tag, logic has over a thousand questions which makes sense to allow better fragmentation into specific parts.
We added propositional-calculus quite some time ago, and it has 171 questions at time of this edit. This was also fine, because it seemed useful.
It seems that a good cleanup is in order. First order.