Decide what the middle words do
Read the sentence with the middle description and once without it. If removing the description changes which person or thing the writer means, the description is essential to identification. If the same subject remains identifiable, the phrase may be an aside. Look at what the reader knows, including whether the text names one individual or asks the reader to choose a subset.
Worked example: two descriptions of a tray
“The trays that have rubber feet sit on the top shelf” describes some trays, identified by their feet. The clause belongs with trays without surrounding commas. Compare “Tray six, the largest tray in the set, sits on the top shelf.” Tray six already identifies it. The description is extra, and a comma marks each place where the sentence enters or leaves the aside.
Look for both edges
An embedded aside normally needs two visible boundaries when you choose commas. A comma on one side only leaves the reader uncertain where the main clause resumes. Find the main subject and verb as a second check. In the tray example they are tray six and sits. Read those words together after mentally taking away the whole aside, including its two commas.
Read the words before a colon
A colon can announce a list after a complete statement. “The case contains three samples: bark, clay, and sand” gives a complete clause before the colon. In “The case contains: bark, clay, and sand,” the verb contains still needs its object, so the list belongs directly after the verb with no colon. This check is for colons in running prose. A form label or a heading can use different conventions.
Transfer check: supplies at a desk
Choose punctuation for “The only visitor desk [a narrow counter near the window] stores three supplies [ ] badges, pens, and maps.” The desk is already identified, so use commas on both sides of the description. The words through supplies form a complete clause, so a colon can introduce the list. Check each boundary independently.
Try it, then explain it.
Complete a short set without looking at the example. After each answer, explain which evidence or rule supports your decision and why the alternatives do not fit.
Sources & authorship
Original OpenExam explanations and examples. References support the concepts; they are not the source of copied questions. Independent educator review is pending.
Purdue OWL: Extended Rules for CommasPurdue OWL: Brief Overview of PunctuationPurdue OWL: Independent and Dependent Clauses