Extracts have been chosen from a wide range of historical and contemporary cases to illustrate the reasoning processes of the courts and to show how legal principles are developed. This chapter reviews the royal prerogative and constitutional conventions, and the relationship between these two sources of constitutional rules. It identifies the various types of prerogative power, and the attempts to place such powers on a statutory basis, including in the Constitutional Reform and Governance Act 2010, and the Fixed-term Parliaments Act 2011. It also examines attempts to codify constitutional practice, including the Crown's personal prerogative of the appointment of the Prime Minister in the Cabinet Manual. The discussion moves on to constitutional conventions as a source of the constitution, their relationship with law, and their nature as rules of political behaviour. The chapter considers the treatment of conventions in the courts, and whether they can obtain legal force. Finally, the chapter discusses the codification of conventions.