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Chet decided to get a huge Chinese dragon tattooed across his shoulder blades
once the stabbing needle pierced his skin, however, Chet quickly changed
his mind,
opting to pierce an ear instead.
Chet quickly changed his mind is a main clause. A main clause can stand alone as a complete sentence. Opting to pierce an ear instead is a present participle phrase. A present participle phrase will begin with an ing word like opting. A legal maneuver in writing is to use a comma to attach a participle phrase to the end of a main clause.