Re: That wicked "which"

"Dr. Engelbert Buxbauum" <engelbert_buxbaum@hotmail.com> wrote or quoted:

What is the difference between 'restrictive' and 'non-restrictive' in
this context?

  A restrictive (or "defining") relative clause serves to pinpoint
  what's being talked about. It's not set off by commas.

  A non-restrictive (or "non-defining") relative clause is
  there to dish out extra info about something already defined.
  It's cordoned off with commas.

  Mary-Claire van Leunen from Digital Equipment Corp said
  Fowler was all about using "that" for defining clauses and
  "which" for non-defining ones. The New Yorker, being the
  grammar sticklers they are, rode that wave for ages. Then
  Strunk and White's "The Elements of Style" came along and
  made this distinction a thing in American English.

  Example sentences:

|All the students that know when to use "which" and "that"
|will pass the quiz. The exam, which took place at the
|beginning of class, was not difficult.

  .