People have trouble with the correct use of that and which.
Writing in 1926, the grammarian H.W. Fowler said the rules are ‘an odd jumble, and plainly show that the language has not been neatly constructed by a master builder’.
Fowler advocated a fairly simple rule (and people who think about these things have largely followed it) – ‘although it would be idle to pretend that it is the practice either of most or of the best writers’ (Fowler again; if you don’t have his Modern English Usage you must buy it now).
- use it where the information in the clause that follows is defining
- in other words, what comes after that is essential to the meaning of the sentence and limits it
- example: She gave me a list of books that had influenced her
- not just any old books, but a limited selection as defined by that
- that is restrictive
- use it where there is no particular limitation in the clause that follows
- what comes after which is not essential to the meaning of the sentence
- example: I always buy her books, which have influenced me greatly
- her books, but the fact they have influenced me is merely descriptive
- which is non-restrictive
Ross Guberman, the US legal writing guru, suggests an easy and workable test: ‘Use “which” if you would pause when reading the sentence out loud. If you wouldn’t pause, use that.’
Next: advice for students and associates