In a common American editing convention, use that for a relative clause needed to identify which thing you mean, and which for a nonessential clause that adds information. “The jacket that has a red cuff is mine” identifies a particular jacket; “My jacket, which has a red cuff, is mine” adds a side detail. Usage differs by dialect, style, and context, so treat this as a useful convention rather than a universal rule.
Does the clause identify the noun?
A relative clause gives information about a noun. If removing the clause would leave the reader unsure which item you mean, the clause is essential to the sentence’s meaning. In the convention often taught in edited American English, that commonly introduces this identifying clause: “The notes that include the final rehearsal time are on the door.” The phrase “that include the final rehearsal time” tells us which notes.
If the reader can already identify the noun and the clause simply adds a detail, which commonly introduces it: “The notes, which include the final rehearsal time, are on the door.” The commas mark the clause as an aside. Purdue OWL’s comma guidance distinguishes essential elements from nonessential ones and notes that clauses beginning with that after nouns are essential in its described convention. [1]
Use the remove-it test carefully
Temporarily remove the clause and reread. “The notes are on the door” still makes grammatical sense, but it may no longer specify which notes. That means the clause may be doing identifying work. By contrast, “The stage manager’s notes are on the door” already identifies a particular set; “which include the final rehearsal time” could be additional information.
The test is about meaning, not whether the sentence remains grammatical. Both versions may stand as sentences after the clause is removed. Ask instead: Does the reader need the clause to pick out the intended noun? If yes, keep the idea tightly attached and, in the target convention, use that without commas. If no, set off the extra information with commas and use which.
A worked example with a room booking
Imagine an invented venue has three rooms. An email says, “The room that faces the courtyard is reserved for the reading.” The clause distinguishes one room from the other two. Now suppose everyone already knows the reading is in the courtyard room. “The room, which faces the courtyard, is reserved for the reading” presents the courtyard-facing feature as a parenthetical detail.
The comma changes the reader’s path through the sentence. Without commas, the clause helps answer “which room?” With commas, the reader can pass over it and retain the main statement. This distinction is useful in event notices, instructions, and prose where a reader must identify the correct object quickly.
Style conventions and accepted variation
Writers do not encounter one uniform rule in every English variety or publication. Some editors apply the that-for-essential, which-for-extra-information pattern consistently. Other usage traditions allow which in restrictive clauses, especially without commas. A house style or assignment may specify a preference; follow it when it does. When no preference is given, choose wording that clearly signals whether the clause identifies or supplements.
You can often avoid a distracting choice by rewriting. “The courtyard-facing room is reserved for the reading” removes the relative clause. Or use “the room with the courtyard view.” A rewrite is especially useful when commas make a sentence hard to follow or when the clause could attach to more than one noun. Purdue’s advice on clarity favors keeping extra phrases from interrupting the main clause when that placement could confuse readers. [2]
Practice with two versions
Try this pair: “The recordings which contain audience questions will be archived” and “The recordings, which contain audience questions, will be archived.” In the first, the clause selects a subset: only recordings with questions. In the second, it implies the recordings are already identified and adds that they contain questions. If the archive includes all recordings, the second meaning is likely intended; if it includes only a selection, the first may be clearer under the convention described here.
When revising, write a short note beside each clause: “identifies” or “adds.” Then check the commas and relative word against your publication’s style. The note forces the real decision into view before a punctuation preference takes over.
