What is the difference between 'Mr' and 'Master'?
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.