Arnold Zwicky over the Language Log on the age-old that/which controversy:
The usual scheme for choosing relativizers is what I’ve called Fowler’s Rule: that in restrictive relatives, which in non-restrictive relatives (it’s more complicated than that, but this is the slogan version).
I’m with Fowler, but there are some authors — Douglas Adams, for one — who consistently uses which in a restrictive sense. Yet more evidence of the futility of a prescriptivist approach, ah guess.
Zwicky makes another point:
I guess I should remind you that in some quarters, “grammar” covers absolutely anything in language that can be regulated: discourse organization, syntax, word choice, morphological forms, stylistic choices, politeness formulas, punctuation, spelling, whatever
This gets on my nerves, too. I tend to use ‘grammar’ to mean ‘syntax’.