Master_(Peerage_of_Scotland)
The heir apparent or heir presumptive to a Scottish peerage is known as a Master, or a Mistress if the heir is female. The heir's style is "The Master of [Peerage]" or "The Mistress of [Peerage]".
If the master is an heir apparent, and the peerage has a subsidiary title that could be used as a courtesy title, then the styling of Master is usually forgone. However, if the person is an heir presumptive, or if the peerage has no subsidiary title, then Master/Mistress is a common styling. However, because the word Mistress is quite archaic, many women choose not to use the style Mistress and instead use the regular styling, e.g. Lady Mary Smith or The Honourable Mary Smith.
Although regarded today as a form of courtesy title, the Mastership is a noble dignity in its own right, and originally conferred rights of attendance in the Parliament of Scotland. As a result, Masters were ineligible for election to the British House of Commons for Scottish constituencies after the Acts of Union 1707. Masters whose elections were declared void on this basis included four elected in the 1708 British general election, who each briefly attended parliament. Members of Parliament would be disqualified upon becoming the eldest (living) son of a Scottish peer, and a by-election would be held for the vacant seat. Therefore, they were denied the right to sit in both houses of the British Parliament. This practice was ended by the Scottish Reform Act 1832, and the Masters could be elected MPs like their English counterparts thereafter.