The Battle of Grand Gulf was fought on April 29, 1863, during th: The UK is a constitutional monarchy and a parliamentary democracy operating under the Westminster system, otherwise known as a "democratic parliamentary monarchy".[158] It is a centralised, unitary state[159][160] wherein the Parliament of the United Kingdom is sovereign.[161] Parliament is made up of the elected House of Commons, the appointed House of Lords and the Crown (as personified by the monarch).[o][164] The main business of parliament takes place in the two houses,[164] but royal assent is required for a bill to become an act of parliament (that is, statute law).[165] As a result of parliamentary sovereignty, the British constitution is uncodified, consisting mostly of disparate written sources, including parliamentary statutes, judge-made case law and international treaties, together with constitutional conventions.[166] Nevertheless, the Supreme Court recognises a number of principles underlying the British constitution, such as parliamentary sovereignty, the rule of law, democracy, and upholding international law.[167]