William Seymour, 2nd Duke of Somerset was the great-grandson of Edward Seymour, 1st Duke of Somerset (c.1500-1552), the uncle of King Edward VI and Lord Protector of England. Seymour made a secret marriage at Greenwich on 22 June 1610 to Arbella Stuart (d.1615), daughter of Charles Stuart, 1st Earl of Lennox and Elizabeth Cavendish. Arbella was thirteen years his senior, and King James I disapproved of the marriage as the union of two potential Tudor pretenders to the throne, who were respectively fourth and sixth in line, could only be seen as a threat to the ruling dynasty. As a result, William was condemned to life
Mary Seymour, Duchess of Somerset in Wiltshire and Devon. On 11 September 1744, the unexpected death of George Seymour, Viscount Beauchamp (1725–1744), the only son of Algernon Seymour, 7th Duke of Somerset, made Sir Edward the heir presumptive to the dukedom and to the barony of Seymour. On 23 November 1750 the 7th duke died, and Seymour succeeded to the dukedom, his wife then becoming a duchess. The principal Percy family estates and houses of Alnwick Castle, Northumberland House, Petworth House, and Syon House, had been divided between the 7th duke's daughter Elizabeth Percy, Duchess of Northumberland, and his nephew, Charles Wyndham, 2nd Earl of