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.
Members of British Parliament disqualified for being a Master
People who currently hold the dignity of Master or Mistress
Lord Alistair Hay, Master of Tweeddale, heir presumptive to the Marquessate of Tweeddale Susan of Mar, Mistress of Mar, heiress presumptive to the Earldom of Mar Alexander David Erskine, Master of Mar and Kellie, heir presumptive to the Earldom of Mar and Kellie Geoffrey Charles Murray, Master of Dunmore, heir presumptive to the Earldom of Dunmore Alexander Cary, Master of Falkland, heir apparent to the Viscountcy of Falkland Christopher Keith Arbuthnott, Master of Arbuthnott, heir apparent to the Viscountcy of Arbuthnott Neil Malcolm Ross Forbes, Master of Forbes, heir apparent to the Lordship of Forbes Katharine Fraser, Mistress of Saltoun, heiress presumptive to the Lordship of Saltoun Jack Fraser, Master of Lovat, heir presumptive to the Lordship of Lovat Francis Sempill, Master of Sempill, heir apparent to the Lordship of Sempill Jago Alexander Elphinstone, Master of Elphinstone, heir presumptive to the Lordship of Elphinstone Laetitia Bruce-Winkler, Mistress of Burleigh, heiress presumptive to the Lordship of Balfour of Burleigh Lewis Edward Palmer, Master of Dingwall, heir apparent to the Lordship of Dingwall (also Great Britain Baron Lucas of Crudwell) William Alexander Hugh Napier, Master of Napier, heir apparent to the Lordship of Napier Alexander Shimi Markus Mackay, Master of Reay, heir apparent to the Lordship of Reay Frederick Carmichael Arthur Hamilton, Master of Belhaven, heir apparent to the Lordship of Belhaven and Stenton James David William Rollo, Master of Rollo, heir apparent to the Lordship of Rollo William Henry Hepburne-Scott, Master of Polwarth, heir apparent to the Lordship of Polwarth