Restrictive covenants (that is, agreements) in real-estate deeds were once used to forbid the buyer from ever selling the property to anyone of another race. These are now illegal, though other kinds of restrictive covenants are very common; in some neighborhoods, they may even tell you what colors you can't paint your house. In grammar, a restrictive clause is one that limits the meaning of something that comes before it. In the sentence "That's the professor who I'm trying to avoid", "who I'm trying to avoid" is a restrictive clause, since it's what identifies the professor. But in the sentence "That's my History professor, who I'm trying to avoid", the same clause is nonrestrictive, since the professor has already been identified as "my History professor". There should always be a comma before a nonrestrictive clause, but not before a restrictive clause.