You know appositives and adjective clauses. Some of them are extra (nonessential): the sentence already tells us who or what. Others are essential: they tell us which one. Extra information gets a pair of commas; essential information gets none.
Key idea
Take the words out. If the sentence still names exactly who or what, the words are extra: set them off with commas. If the sentence no longer tells which one, they are essential: no commas.
Why it works
The commas change the meaning. “My sister who lives in Texas is a nurse” suggests you have more than one sister and are pointing to the one in Texas. “My sister, who lives in Texas, is a nurse” says you have one sister, and by the way she lives in Texas. Strunk’s third rule (1918): “Enclose parenthetic expressions between commas.”
Worked example: A name already identifies
“Mount Rainier which is in Washington is a volcano.”
- Take out “which is in Washington”: “Mount Rainier is a volcano.”
- The name already tells us which mountain, so the clause is extra.
- Put commas on both sides of it.
Answer: Mount Rainier, which is in Washington, is a volcano.
Worked example: An essential appositive
“The author Louisa May Alcott wrote Little Women.”
- Take out the name: “The author wrote Little Women.” Which author?
- The name tells which one, so it is essential.
- No commas.
Answer: No commas: “The author Louisa May Alcott wrote Little Women.”
Common mistake
Using only one comma (“My brother, who lives in Ohio is visiting”). Extra information in the middle of a sentence needs a pair. Which or that? American usage guides prefer that for essential clauses and which, with commas, for extra ones. Which in an essential clause (“the book which I lent you”) is standard too, especially in British English; it isn’t an error.