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Where did you hear these terms at? Generally, I hear people talking about singing open and it's opposite which would be covered. When you sing "open", generally people refer to using wider vowels. When you sing "covered", people generally talk about singing with more rounded vowels. I also hear people talk of an "open throat" and a "closed throat". When you have an "open throat", you're free from constriction; when you sing with a "closed throat", they mean you're singing with constriction.