Use commas to set off nonrestrictive modifiers. Anonrestrictive modifier is usually introduced by which and contains information thatis not essential to establishing the meaning of what it modifies. See Infectious diseases, which antibiotics and vaccines once promised to banishfrom our shores, have returned with a vengeance.
--Jonathan Tucker, "Invaders from the Rainforest: Review of The HotZone by Richard Preston," Technology Review
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