Missing Commas Invent Sisters
My sister who lives in Tulsa is a doctor. My sister, who lives in Tulsa, is a doctor. One of those sentences tells the reader I have at least two sisters. The difference is a pair of commas.
Grammar calls them restrictive and nonrestrictive, which helps nobody. In plain terms, a clause without commas is a sorting instruction. It tells the reader which one you mean. A clause with commas is a bonus fact about someone the reader already knows.
“We ate the cake that fell on the floor” means several cakes existed and you chose the floor one. “We ate the cake, which fell on the floor” means there was one cake, gravity happened, and you ate it anyway.
Readers apply this rule without knowing it exists. Write “the cop who had testified against him” and you’ve told them there were other cops. Maybe you meant that. Maybe you invented two extra officers your plot has to support.
The fix is mechanical. Search your draft for every which. Delete the clause and reread. If the reader still knows exactly which person you mean, the clause was a bonus; put the commas back. If the sentence can no longer pick one out of a lineup, the clause is doing sorting work; take the commas out.
