Earl Fitzwilliam was a title in both the Peerage of Ireland and the Peerage of Great Britain held by the head of the Fitzwilliam family. The Fitzwilliams acquired extensive holdings in the south of the West Riding of Yorkshire, largely through strategic alliances through marriage. In 1410, Sir John Fitzwilliam of Sprotborough, who died in 1421, married Margaret Clarell, daughter of Thomas Clarell of Aldwark, the descendant of a major Norman landholding family. This is how the Fitzwilliams acquired the Clarell holdings.