Traditionally, "Mister (Mr)" was a title used to address married men, while "Master (Mstr)" was used to address unmarried men.
Over time, the use of "Mister" expanded to include unmarried men, and "Master" became restricted to boys and young men who had not yet entered society.
Even this is quite rare today, and is mostly used in extremely formal written correspondence.
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Mister is a title that is not properly a form of address all by itself; that is, it is not polite to address someone simply as "mister." Mister Jones, yes, but not just plain "mister." For direct address, we use "sir" if we are being very formal or if we work behind a counter.