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.