Re: Message from Internet

[Kent Newton <KentN -at- METRIX-INC -dot- COM>]

According to the American Heritage Dictionary (Third Edition), "that"
should be used as only restrictive while "which" can be used as both
restrictive and non-restrictive.  It also notes that the use of "that" to
introduce a non-restrictive clauses was once quite common, but should be
avoided in careful writing today.  Below is the complete Usage Note from
the dictionary.

================  American Heritage Dictionary Usage Note
========================

The standard rule is that that should be used only to introduce a
restrictive (or +defining+) relative clause, which serves to identify the
entity being talked about; in this use it should never be preceded by a
comma. Thus, we say "The house that Jack built has been torn down," where
the clause "that Jack built" tells which house was torn down, or "I am
looking for a book that is easy to read," where "that is easy to read"
tells what kind of book is desired. Only which is to be used with
nonrestrictive (or +nondefining+) clauses, which give additional
information about an entity that has already been identified in the
context; in this use, which is always preceded by a comma. Thus, we say
"The students in Chemistry 10 have been complaining about the textbook,
which (not that) is hard to follow." The clause "which is hard to follow"
does not indicate which text is being complained about; even if it were
omitted, we would know that the phrase "the textbook" refers to the text
in Chemistry 10. The use of that in nonrestrictive clauses like this,
though once common in writing and still frequent in speech, is best
avoided in formal style. o Some grammarians have argued that symmetry
requires that which should be used only in nonrestrictive clauses, as
that is to be used only in restrictive clauses. Thus, they suggest that
we should avoid sentences such as "I need a book which will tell me all
about city gardening," where the clause "which will tell me all about
city gardening" indicates which sort of book is needed.  Such use of
which is useful where two or more relative clauses are joined by and or
or, as in "It is a philosophy in which the common man may find solace and
which many have found reason to praise." Which is also preferred to
introduce a restrictive relative clause when the preceding phrase itself
contains a that, as in "I can only give you that which I don't need (not
that that I don't need)" or "We want to assign only that book which will
be most helpful" (preferred to that book that will be most helpful). o
That may be omitted in a relative clause when the subject of the clause
is different from the referent of the phrase preceding the clause. Thus,
we may say either "the book that I was reading" or "the book I was
reading," where the subject of the clause ( I) is not the referent of the
phrase the book. Omission of that in these cases has sometimes been
described as incorrect, but the practice is extremely common and has
ample precedent in reputable writing. o There have also been occasional
objections to the omission of that in its use to introduce a subordinate
clause, as in "I think we should try again." But this usage is entirely
idiomatic and is in fact favored with some of the verb phrases that can
introduce such clauses: thus, one would more normally write "I take it
she has passed the test" than "I take it that she has passed the test."
That should not be omitted, however, when the subordinate clause begins
with an adverbial phrase or any element other than the subject: "She said
that under no circumstances would she allow us to skip the meeting." "The
book argues that eventually the housing supply will increase." This last
sentence would be ambiguous if that were omitted, since the adverb
eventually could then be construed as modifying either argues or will
increase. See note at doubt this whatever which who.

================= End Usage Note ==========================

While I hope that puts an end to all questions about the use of "that"
and "which,"  I doubt it will.

Kent Newton
Senior Technical Writer
Metrix, Inc.
kentn -at- metrix-inc -dot- com