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 TweeddaleLady Susan Helen of Mar, Mistress of Mar, heiress presumptive to the Earldom of MarAlexander David Erskine, Master of Mar and Kellie, heir presumptive to the Earldom of Mar and KellieJames Reginald Drummond, Master of Perth, heir presumptive to the Earldom of PerthGeoffrey Charles Murray, Master of Dunmore, heir presumptive to the Earldom of DunmoreLucius Alexander Plantagenet Cary, Master of Falkland, heir apparent to the Viscountcy of FalklandJohn Keith Oxley Arbuthnott, Master of Arbuthnott, heir apparent to the Viscountcy of ArbuthnottNeil Malcolm Ross Forbes, Master of Forbes, heir apparent to the Lordship of ForbesKatherine Fraser, Mistress of Saltoun, heiress presumptive to the Lordship of SaltounHugh Alastair Joseph Fraser, Master of Lovat, heir presumptive to the Lordship of LovatFrancis Sempill, Master of Sempill, heir apparent to the Lordship of SempillJago Alexander Elphinstone, Master of Elphinstone, heir presumptive to the Lordship of ElphinstoneVictoria Bruce-Winkler, Mistress of Burleigh, heiress presumptive to the Lordship of Balfour of BurleighLewis 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 NapierÆneas Simon Mackay, Master of Reay, heir apparent to the Lordship of ReayFrederick Carmichael Arthur Hamilton, Master of Belhaven, heir apparent to the Lordship of Belhaven and StentonJames David William Rollo, Master of Rollo, heir apparent to the Lordship of RolloWilliam Henry Hepburn-Scott, Master of Polwarth, heir apparent to the Lordship of Polwarth