<P> The use of Master as a prefixed title was, according to Leslie Dunkling, "until recently...a way of addressing politely a boy who was too young to be called' Mister' ." It was used as a title for the eldest son only; younger sons had no form of address . </P> <P> Master is used sometimes, especially up to circa late - 19th century, especially in the UK to describe the male head of a large estate or household who employs domestic workers . </P> <P> The heir to a Scottish lordship, barony or viscountcy is given the style or dignity Master of followed by his father's title . For instance, the heir of Lord Elphinstone is known as the Master of Elphinstone . </P> <P> Nancy Tuckerman of the Amy Vanderbilt Complete Book of Etiquette writes that in the USA, unlike the UK, a boy can be addressed as master only until age 12, then is addressed only by his name with no title until he turns 18, when he takes the title of Mr., although it is not improper to use Mr. if he is slightly younger . In former times the African - American slaves addressed their owners as master, and they pronounced it in their dialect as "massa". Robert Hickey, deputy director of the Protocol School of Washington, states that "use of Master (as) an honorific when addressing boys is considered old fashioned outside of conservative circles ." </P>

When does it change from master to mr