The United Kingdom has three distinctly different legal systems, each of which derives from a particular geographical area for a variety of historical reasons: English law (in the joint jurisdiction of England and Wales), Scots law, Northern Ireland law, and, since 2007, calls for a fourth type, that of purely Welsh law as a result of Welsh devolution. In fulfilment of its former EU treaty obligations, European Union directives had been transposed into the UK legal system on an ongoing basis by the UK parliament. Upon Brexit, non-transposed EU law (such as regulations) was transplanted into domestic law as "retained EU law", with an additional period of alignment with EU law during the transition period from 31 January to 31 December 2020.