Adjective Clauses: Essential or Non-Essential
An ADJECTIVE CLAUSE is essential if it is necessary or required to make sense of the sentence. Essential ADJECTIVE CLAUSES get zero commas.
An ADJECTIVE CLAUSE is non-essential if its information is not critical to the meaning of the sentences. Non-essential ADJECTIVE CLAUSES are set off with commas.
The woman who sells silver jewelry is not here on Mondays.
WHO SELLS SILVER JEWELRY is essential.
LeeAnn, who sells jewelry, is not here on Mondays.
WHO SELLS SILVER JEWELRY is non-essential.
Clauses beginning with THAT are usually essential.
Clause beginning with WHICH are usually non-essential.