During the Roman occupation of Britain, the area of present-day England and Wales was administered as a single unit, with the exception of the land to the north of Hadrian's Wall – though the Roman-occupied area varied in extent, and for a time extended to the Antonine/Severan Wall. At that time, most of the native inhabitants of Roman Britain spoke Brythonic languages, and were all regarded as Britons, divided into numerous tribes. After the conquest, the Romans administered this region as a single unit, the province of Britain. Long after the departure of the Romans, the Britons in what became Wales developed their own system of law, first codified by Hywel Dda when he was king of most of present-day Wales; in England Anglo-Saxon law was initially codified by Alfred the Great in his Legal Code,. However, after the Norman invasion of Wales in the 11th century, English law came to apply in the parts of Wales conquered by the Normans. In 1283, the English, led by Edward I, with the biggest army brought together in England since the 11th century, conquered the remainder of Wales, then organised as the Principality of Wales. This was then united with the English crown by the Statute of Rhuddlan of 1284. This aimed to replace Welsh criminal law with English law. Welsh law continued to be used for civil cases until the annexation of Wales to England in the 16th century. The Laws in Wales Acts 1535–1542 then consolidated the administration of all the Welsh territories and incorporated them fully into the legal system of the Kingdom of England. Prior to 1746, it was not clear whether a reference to "England" in legislation included Wales, and so in 1746 Parliament passed the Wales and Berwick Act. This specified that in all prior and future laws, references to "England" would by default include Wales. The Wales and Berwick Act was repealed by the Welsh Language Act in 1967, although the statutory definition of "England" created by that Act still applies for laws passed before 1967. In new legislation since then, what was referred to as "England" is now "England and Wales", while subsequent references to "England" and "Wales" refer to those political divisions.
England and Wales are treated as a single unit for some purposes, because the two form the constitutional successor to the former Kingdom of England. The continuance of Scots law was guaranteed under the 1706 Treaty of Union that led to the Acts of Union 1707, and as a consequence English lawand after 1801, Irish lawcontinued to be separate. Following the two Acts of Union, Parliament can restrict the effect of its laws to part of the realm, and generally the effect of laws, where restricted, was originally applied to one or more of the former kingdoms. Thus, most laws applicable to England also applied to Wales. However, Parliament now passes laws applicable to Wales and not to England, a practice which was rare before the middle of the 20th century. Examples are the Welsh Language Acts 1967 and 1993 and the Government of Wales Act 1998. Measures and Acts of the Senedd Cymru apply in Wales but not in England. Following the Government of Wales Act, effective since May 2007, the Senedd can legislate on matters devolved to it. Following a referendum on 3 March 2011, the Senedd gained direct law-making powers, without the need to consult Westminster. This was the first time in almost 500 years that Wales had its own powers to legislate. Each piece of Welsh legislation is known as an Act of the Senedd. of England and Wales
For a company to be incorporated in the United Kingdom, its application for registration with Companies House must state "whether the company's registered office is to be situated in England and Wales, in Scotland or in Northern Ireland", which will determine the law applicable to that business entity. A registered office must be specified as "in Wales" if the company wishes to use a name ending cyfyngedig or cyf, rather than Limited or Ltd. or to avail itself of certain other privileges relating to the official use of the Welsh language.
Outside the legal system, the position is mixed. Some organisations combine as "England and Wales", others are separate.