Hertford, William Seymour, 1st marquess and 2d earl of (härˈfərd, härtˈ–) [key], 1588–1660, English nobleman; great-grandson of Edward Seymour, duke of Somerset, and grandson of Lady Catherine Grey, through whom he had a claim to the throne. His secret marriage (1610) to Arabella Stuart, cousin of James I, enraged the king, and he was imprisoned. He escaped and fled the country, but returned in 1616 after Arabella's death. Made a privy councillor (1640) and marquess of Hertford (1640), he fought for Charles I in the civil war. He received his ancestor's dukedom of Somerset at the Restoration (1660).