I may be being a bit progressive about this, but it seems to me that the English are somewhat more lax about the that/which distinction than those in the US. I think that you will find that the latter hold that "which" be limited to parenthetic remarks (i.e. those delimited by commas or other means).
But I should be careful writing about comma usage 'cos we have a disagreement over on t'other site.
Tone
P.S. Anyways, they might not be "best", just lucky!
I would have said that that is used to introduce a defining relative clause and which is used to introduce a non-defining clause; however, after reading this in my style guide, I probably do not need to follow such a hard line:
The choice between that and wh- relatives is sometimes said to depend on whether it prefaces a restrictive or nonrestrictive relative, with that for the restrictive type and which for the other (see previous entry). This is an oversimplification of Fowler's original suggestion that they could be used that way, though even he admitted: "It would be idle to pretend it was the practice either of most or of the best writers." Later style commentators note that while which is indeed preferred for nonrestrictive relative clauses, both that and which can be found with the nonrestrictive type.
The previous entry referred to above says this:
Relatve clauses which serve to define or identify something have often been called "restrictive" - which makes "nonrestrictive" all the other kinds which describe or evaluate or add writers' comments. (Alternative names are defining and nondefining relatives.) ... The distinction between a relative clause which defines and one which does something else is not always ... clear-cut ... and grammarians note ambiguous cases.
Geoff's reply sums up the advice contained in most of my grammars / style guides (UK, US, and AU), and Tone's suggestion that the British are laxer than Americans in this matter aligns with much of my experience. Here in Oz, inconsistency and uncertainty are the order of the day.
In real life -- as compared with theory -- I have found general confusion and inconsistency from writers both sides of the Atlantic (and equator). Christopher Isherwood (UK born, raised, and first published; later US citizen and US published) uses which almost to the complete exclusion of that (regardless whether a particular piece was written, or published, in UK or US). Gore Vidal (US) is consistent in following the convention as described by Strunk and White (see below); Robert Graves (UK) consistently uses only which; E M Forster (UK) follows the Strunk and White style; Douglas Adams (UK) mixes them indiscriminately; Thomas Keneally (AU) uses mostly (though not exclusively) which.
In school (AU) in the '50s and '60s we were taught, quite strictly, that that was "common" and should be avoided whenever which could be employed: another of those over-simplified (and frequently wrong) notions held and imparted by school teachers.
I have a strong preference, in my own usage, for that to introduce a restrictive clause (no commas) and which for a non-restrictive clause (separated by commas). Of course, nothing is set in concrete, and there are occasions where I don't follow that convention (e.g. in the sequence: "You shall have that which you desire ").
I like Strunk and White's statement on the matter:
The use of which for that is common in written and spoken language [...]. Occasionally which seems preferable to that [...]. But it would be a convenience to all if these two pronouns were used with precision. Careful writers, watchful for small conveniences, go which-hunting, remove the defining whiches, and by so doing improve their work.
“STYLE is knowing who you are, what you want to say, and not giving a damn.” — Gore Vidal “STYLE is knowing what sort of play you're in.” — Sir John Gielgud “Politicians are not born; they are excreted.” ― Marcus Tullius Cicero “Be yourself; everyone else is already taken.” ― Oscar Fingal O’Fflahertie Wills Wilde
Relatve clauses which serve to define or identify something have often been called "restrictive" - which makes "nonrestrictive" all the other kinds which describe or evaluate or add writers' comments. (Alternative names are defining and nondefining relatives.) ... The distinction between a relative clause which defines and one which does something else is not always ... clear-cut ... and grammarians note ambiguous cases. [my bold]
Shouldn't those whiches be thats? (she asked innocently!)
Sue
"He who dares not offend cannot be honest" Thomas Paine
In real life -- as compared with theory -- I have found general confusion and inconsistency from writers both sides of the Atlantic (and equator). Christopher Isherwood (UK born, raised, and first published; later US citizen and US published) uses which almost to the complete exclusion of that (regardless whether a particular piece was written, or published, in UK or US). Gore Vidal (US) is consistent in following the convention as described by Strunk and White (see below); Robert Graves (UK) consistently uses only which; E M Forster (UK) follows the Strunk and White style; Douglas Adams (UK) mixes them indiscriminately; Thomas Keneally (AU) uses mostly (though not exclusively) which.
Well, the only one that / which counts as a real author ;D is Douglas Adams, so I'll just keep mixing it up.
A learned blockhead is a greater blockhead than an ignorant one (Benjamin Franklin)
Shouldn't those whiches be thats? (she asked innocently!)
I was quoting, Sue; but you make a good point, I think.
jig1,
Perhaps our explanations lacked supporting examples, but the point Sue makes illustrates what Verbivore and I were saying:
1. Strictly speaking, I think the whiches should be thats as each relative clause could be considered to be defining/restrictive.
2. Hard-liners would insist that defining/restrictive clauses should start with that, but here is a writer speaking on the subject of which/that who, perhaps, does not support the idea it is a rule to which one must rigidly adhere.
3. Who is to say what is defining/restrictive and what is non-defining/non-restrictive?
Yes, I realised that, Geoff. That was why I mentioned it, as it seemed particularly ironic. If it had been your own preferred use, I'd have had nothing to say. Like Douglas Adams (and me, usually), you may do as you wish, however an explanation that patently breaks the rule it is trying to explain is not particularly helpful!
Sue
"He who dares not offend cannot be honest" Thomas Paine
And very nearly did. But someone else writ wot I would'a writ afore I could put fingers to keys.
And as for volcanic ash -- I knew no good would come of this new-fangled civil aviation. They should have left planes to the guvmint and military and made civilians go by boat! (Thus reducing the spread of infections, making the world a generally better place, and saving the Twin Towers.)
Tone
"Does Magna Carta mean nothing to you? Did she die in vain?" "I try never to burn my bridges -- but an arsonist dogs my footsteps."