The heir apparent or heir presumptive to a Scottish peerage is known as a Master, or 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 subsidiary titles 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 titles, 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 dignity in its own right, and originally conferred rights of attendance in the Parliament of Scotland. As noblemen, Masters were ineligible for election to the House of Commons of Great Britain for Scottish constituencies: Masters whose elections were declared void on this basis included Lord Johnstone (the Master of Annandale), Lord Haddo (the Master of Aberdeen), Lord Strathnaver (the Master of Sutherland) and the Master of Sinclair. Lord Elcho was excluded from Parliament in 1787 on the grounds that he had become Master of Wemyss, though in fact the peerage was under forfeiture.
People who currently hold the title Master or Mistress:
Lord Alistair James Montagu Hay, Master of Tweeddale, heir presumptive to the Marquessate of Tweeddale
Lady Susan Helen 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
James Reginald Drummond, Master of Perth, heir presumptive to the Earldom of Perth
Geoffrey Charles Murray, Master of Dunmore, heir presumptive to the Earldom of Dunmore
Lucius Alexander Plantagenet Cary, Master of Falkland, heir apparent to the Viscountcy of Falkland
John Keith Oxley 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
Katherine Fraser, Mistress of Saltoun, heiress presumptive to the Lordship of Saltoun
Hugh Alastair Joseph 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
Victoria 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
Æneas Simon 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 Hepburn-Scott, Master of Polwarth, heir apparent to the Lordship of Polwarth
