#sticky_and_tricky Which or That and Punctuation in Relative Clauses. Part 1 A relative clause is a clause that adds information about the subject or an object in the main clause. A relative clause is often introduced by relative pronouns that and which. 1. Punctuation in non-defining clauses A non-defining clause provides supplementary information about the object it describes; otherwise, it does not provide any information that helps identify the object that is denoted in the main clause. Non-defining clauses can be introduced by relative pronouns which, whose, who and others but not that and are always marked off by a comma or commas, depending on their position in a sentence. Examples: The red carpet on the floor, which rests on brick supports and wooden beams, helps decorate the room. When Amanda saw a meteorite falling through the Earth's atmosphere, which primarily consists of nitrogen and oxygen, she took a photo of it with her photo camera. Jack proudly put the ten pounds, which he had been meticulously saving with the odd errands he had been getting here and there, in a jar his mom had given him for his savings. When the boys were playing with the football in Mike's backyard, they found a human arm, which must have been hastily buried there a few days ago. 2. Punctuation in defining clauses Defining clauses provide information about an object that is indispensable in identifying the object that is denoted in the main clause. They can be introduced by both that and which and are not marked off by any punctuation marks. Examples: I have stopped in Venice at the hotel which Mark recommended to me. Next time, don't forget to do the exercises which you haven't handed in today. When collectors pay silly money for musical instruments which famous musicians have owned and played they are often buying instruments of very regular quality.