This dissertation is about the Anglo-French dual monarchy during the reigns of Henry VI and Edward IV. It seeks to understand what the dual monarchy meant for contemporaries. It looks at Yorkist perceptions of the dual monarchy and how they used it as a political weapon for destabilising Henry VI’s legitimacy and establishing the legitimacy of Edward IV. Dual or composite monarchies in the late medieval period have not been widely explored. Only recently have historians sought to understand these political concepts as they continue to be relevant today. This dissertation explores Yorkist perceptions of the Anglo-French dual monarchy by using chronicle sources. These are sources that historians of political thought have traditionally neglected, but which are now beginning to be considered. This study follows this new trajectory. The dual monarchy was a significant political concept in the fifteenth century. It was an established system of government and contemporaries understood how it should function. The Yorkists used the dual monarchy to delegitimise Henry VI by emphasising the failure of Henry’s advisors to manage it. The Yorkists also used a claim to the dual monarchy to reinforce their hereditary right to the throne which was a key means of legitimation in the later Middle Ages.