That introduces information the sentence needs. Which introduces extra information and usually takes a comma. Delete the clause. If you still know which thing you mean, use which with commas. If you do not, use that with none. The deletion test is the same one in essential and nonessential clauses.
Needed versus extra
The calculator that died was the backup. Without the clause, you do not know which calculator. No commas, that. My calculator, which died in section two, was the backup. You already know which one. Commas, which.
People still take who
Do not use which for a person. That fork is in that versus who. The relative-pronoun map is in relative pronouns. Comma pairs around extra information are also in ACT comma rules.
The bottom line
Needed clause: that, no commas. Extra clause: which, commas on both sides.
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