My well-worn copy of Strunk and White[1] encourages me to go on a “which hunt”, replacing uses of the word “which” with “that”. I remember reading this a long time ago, and since then I’ve made it a practice. But over time I forgot why I was doing it.
The “why” relates to what are known as restrictive and non-restrictive clauses in sentences. Restrictive clauses refer specifically to an object or event. An example of a restrictive clause in a sentence is “The satellite that no longer functions is still in orbit”. In this sentence the writer is referring to a specific satellite that is broken, but still in orbit. “That” is used.
A non-restrictive clause adds information about an event or an object. And example of this is “The Landsat orbiter, which no longer functions, is still in orbit.”. Here the writer is talking about the Landsat satellite and telling the reader that, by the way, it no longer functions. “Which” is appropriate here.
In my life as a copyeditor, I have, from time to time, made this mistake. The sharp authors out there have called
John @ ProofreadCanada
[1] Struck Jr., W., and E. White. 1979. The Elements of Style. Third Edition. Simon & Schuster, Needham Heights, MA.