What is a Rosin Bag?

A rosin bag is a small burlap or canvas bag that is filled with rosin powder, a semi-sticky to sticky substance that helps pitcher’s get a better grip on the baseball. MLB wisely allows pitchers to use rosin to acquire a dry grip for the sake of hitters’ safety.

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A pitcher's mound with a rosin bag laying on it.
That rosin bag looks rather lonely.