Adam Clarke on Proverbs 17:28

Even a fool - He is counted wise as to that particular. He may know that he cannot speak well, and he has sense enough to keep from speaking. He is, as to that particular, a wise fool.

A man may be golden-mouthed and silver-tongued in eloquence; but to know when and where to speak and to be silent, is better than diamonds. But who that thinks he can speak well can refrain from speaking? His tongue has no rest.