Many writers think a sentence is made more elegant
by a restrictive clause introduced by which. But the usage that Fowler,
Follett, Strunk and White, and many other arbiters of usage prefer is
to use that for restrictive clauses, which for nonrestrictive. Such usage
at least shows that the writer knows the difference between restrictive
and nonrestrictive clauses.
|The book which I wrote in 1994 is about
|The book that I wrote in 1994 is about
The use of that leaves no question about whether the
clause is restrictive. And reserving which for nonrestrictive clauses
leaves no question about which which clauses are to be punctuated. Note,
however, the exception of restrictive clauses that begin with a preposition:
the manner in which she does things. Note, too, that the exception can
be circumvented by rewriting: the way she does things.