Based on a writing tip I wrote recently for my work colleagues…
- Use ‘that’ if the phrase is integral to the meaning of the sentence (i.e. a ‘restrictive’ or ‘essential’ clause).
- Use ‘which’ if the sentence can stand alone without the phrase (i.e. ‘non-restrictive’ or ‘non-essential’ clause); a ‘which’ clause typically expands on or enhances the information already given.
- ‘Which’ typically follows a comma, whereas ‘that’ rarely does.
- A good test for when to use ‘that’ or ‘which’ is to remove the clause—if the sentence still makes sense, use ‘which’; if it doesn’t, use ‘that’.
One area where I make a lot of corrections when editing a document is the use of ‘that’ or ‘which’. There are hundreds of web pages devoted to discussing the differences, some very technical – I’ve listed a few below.
Your use of ‘that’ or ‘which’ depends on whether your clause is restrictive or not.
Examples of non-restrictive clauses (i.e. they could be removed without changing the meaning of the sentence) where you would use ‘which’ are shown in red:
- A subset of the Emergency Response Plan is the Marine Oil Pollution Plan, which describes the [company's] response to an oil spill.
- Maps and detailed information have been compiled for [location], which detail the beaches, vehicle access tracks, habitats, vegetation communities, mangroves, and other sensitive areas.
- The largest vessels of the fleet, which represent the greatest potential spill volumes, always operate with support vessels nearby.
- All bores, which include production, exploration, and purpose-drilled monitoring bores, are to be sealed or capped appropriately to prevent ingress of fauna.
Examples of restrictive clauses (i.e. the clause is integral to the sentence’s meaning) where you would use ‘that’ are shown in red:
- Known active warrens occupied by [animal] are to be trapped by [company/government department] within one week prior to clearing, so as to remove individuals that occupy the warren and relocate them elsewhere (under licence) as agreed between [company] and [government department]. (NOTE: This one is restrictive as you’re only referring to individuals that occupy the warren, not all individuals.)
- For sites that are cleared in progressive stages, fauna searches should be undertaken at each stage of clearance. (NOTE: This one is restrictive as you’re only referring to specific types of sites, not all sites.)
- Techniques that wash oiled sand into the lower intertidal and subtidal zones should be avoided. (NOTE: This one is restrictive as you’re only referring to specific types of techniques, not all techniques.)
[And no, I won't comment on the use of passive voice in these examples...]
[Links last checked May 2013]