Commas and Relative Clauses

Sometimes you set off relative clauses with commas, but when the clause is ESSENTIAL to the meaning of the sentence, you do not use commas. (This is also true for other kinds of clauses, not just relative clauses.) Here are some examples:

• ESSENTIAL. Arachne was a human weaver who challenged the goddess Athena to a weaving contest.
  (The "who" clause is an essential part of the sentence; without that clause, the sentence does not convey its main idea.)
• NON-ESSENTIAL. Arachne, who was a talented weaver, challenged the goddess Athena to a weaving contest.
  (The "who" clause provides useful information, but the clause is not essential to the sentence; if you leave it out, the sentence still conveys the main idea.)

For more about essential v. non-essential clauses, see Purdue's OWL.

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Here you see Arachne's contest with Athena; visit the Bestiaria for more information.

(image source)