Seattle is a beautiful city.  It has many beautiful lakes and parks.

  1. How many Seattle are there?
    It is unique.  It is already specific.  You are not choosing it out of a group.
  2. If the noun that you are describing in unique, you need to use commas to set off the adjective clause.

    Seattle, which has many beautiful lakes and parks, is a beautiful city.



  1. You can't use "that" when you have a nonessential adjective clause.
  2. You can't delete the relative pronoun.

Dan has three brothers.  Bill lives in Los Angeles.  Jeff lives in Port Orchard.  Gary lives in Portland.  Bill is a doctor.  Jeff is an accountant.  Gary is a forester.

My brother who lives is Los Angeles is a doctor--not my brother who lives in Portland.  (In this situation you are trying to specify one brother out of many brothers.)

Bill, who lives in Los Angeles, is a doctor.  (In this case you know the Bill that I am talking about.  There is only one Bill.  Your are not choosing out of a group.

Proper Nouns are usually already specific.  This means that you need commas.

Can you list some proper nouns?

Be careful with generic nouns.  Sometimes we will put an "s" on a noun when we are referring to all in the group.  The group becomes one.  For example, 

Elephants are endangered species.  They are the largest land mammals.
Elephants are very valuable animals to farmers and foresters. They are healthy and strong.



1.  Non-restricted or non-essential adjective clauses need commas.

2.  If the noun you are describing in unique or a proper noun, you usually need to you use a comma to set off the adjective clause.

3.  If you are trying to specify "which one" out of group of many, then you shouldn't use commas. If you can answer the question Which one?  Then you do not need to use a comma.