The 1st Marquess had been created Baron Ailsa in the Peerage of the United Kingdom on 12 November 1806.

The family's seats were Cassillis House[6] and Culzean Castle, near Maybole, Ayrshire.

The latter, William [Douglas], Earl of Ruglen and March later Duke of Queensberry, was son and heir of William [Douglas], Earl of March, by his wife Anne [Hamilton], suo jure Countess of Ruglen, 1st dau.

The entail of 1759 was however upheld, though by a narrow majority, in the Court of Session, and confirmed, on appeal, by the House of Lords.

Sir Thomas Kennedy, Bart., claimed the said titles as heir male, and the two petitions having been laid before the House of Lords, it was adjudged, 27 January 1762, that the latter had a right "to the honour and dignity of Earl of Cassillis as heir male of the body of David, the 1st Earl of Cassillis, and [to that] of Lord Kennedy as heir male of the body of Gilbert, the 1st Lord Kennedy.