Some grammarians want to restrain "which" to non-restrictive clauses, as opposed to restrictive clauses; for restrictive clauses, they prescribe "that". It seems that native English speakers very often violate this rule. As a consequence, the rule seems to fail to describe prevalent patterns of actual usage.
Promulgators of the rule: Fowlers as a wish rule, White (but not Strunk) as an actual rule, as per G. K. Pullum.
For terminology, see Google Ngram Viewer for restrictive clauses, nonrestrictive clauses, defining clauses, essential clauses, nonessential clauses.
| Post | Author | Date | Venue |
|---|---|---|---|
| Which vs that? I have numbers! | Geoffrey K. Pullum | September 19, 2004 | Language Log |
| Which vs. that: a test of faith | Mark Liberman | September 20, 2004 | Language Log |
| Which vs. that: integration gradation | Mark Liberman | September 23, 2004 | Language Log |
| Reddit blewit | Mark Liberman | December 24, 2012 | Language Log |
| which/that | Arnold Zwicky | October 23, 2011 | Arnold Zwicky's Blog |
| The myth that which is banned from integrated relatives | Geoffrey K. Pullum | 11/28/2012 | Geoffrey K. Pullum homepage |
| The Land of the Free and The Elements of Style | G. K. Pullum | 2010 |