Between relative clauses which explain the antecedent, or which introduce a new thought.
The type, which was badly worn, was not fit for the job.
If the relative clause limits the meaning of the antecedent, but does not explain it and does not add a new thought, the comma is not used.
He did only that which he was told to do.
7.

To separate parenthetical or intermediate expressions from the context.
The school, you may be glad to know, is very successful.
The books, which I have read, are returned with gratitude.
He was pleased, I suppose, with his work.
If the connection of such expressions is so close as to form one connected idea the comma is not used.
The press nearest the south window is out of order.
If the connection of such expressions is remote, parentheses are used.
The Committee (appointed under vote of April 10, 1909) organized and proceeded with business.
8.

To separate the co-ordinate clauses of compound sentences if such clauses are simple in construction and closely related.
He was kind, not indulgent, to his men; firm, but just, in discipline; courteous, but not familiar, to all.
9.

To separate quotations, or similar brief expressions from the preceding part of the sentence.
Caesar reported to the Senate, "I came, I saw, I conquered." The question is, What shall we do next?
10.

To indicate the omission of the verb in compound sentences having a common verb in several clauses.
One man glories in his strength, another in his wealth, another in his learning.
11.