The general election on 12 December will determine who is in charge at Westminster, but the UK government's power to make laws across the land is limited.
Many functions have been passed to Scotland, Wales and Northern Ireland, which have their own elected bodies. However, they don't all share the same level of authority.
How did it happen?
There have long been important distinctions in the way different parts of the UK work - for example, the separate education and legal systems in Scotland. But in 1997, devolution - the transfer of some powers from Westminster - entered a new phase.
That year, referendums were held in Scotland and Wales, with both parts of Ireland following in 1998.
These resulted in the creation of the Scottish Parliament, the National Assembly for Wales and the Northern Ireland Assembly.
Another major shake-up came in the form of 2014's Scottish referendum. Although voters said "No" to full independence for Scotland, the period leading up to the vote led to an intense debate over what Scotland should be able to decide for itself.
The debate resulted in more powers being handed to the Scottish Parliament - and is likely to influence how the process of devolution continues, not just in Scotland, but across the UK.
Which powers are devolved?
The table below gives an overview of the main powers of the Northern Irish and Welsh assemblies, and the Scottish Parliament.
* Scotland has always had its own legal and education system
Which powers are not devolved?
The UK government, led by the prime minister in Downing Street, is responsible for national policy on all powers which have not been devolved - known usually as "reserved powers".
The main areas which are reserved to Westminster are:
- The constitution
- Defence and national security
- Drug policy
- Foreign policy
- Immigration and citizenship
- Social security (devolved in Northern Ireland and partly in Scotland)
- Most forms of tax (but not in Scotland and with exceptions in Wales)
The Scottish Parliament sits at Holyrood in Edinburgh and is made up of 129 elected Members of the Scottish Parliament (MSPs), who scrutinise and pass bills in various areas of its many devolved responsibilities.
In addition to the main devolved powers, the parliament can legislate on tourism, planning, natural and built heritage, sport and the arts, statistics, public registers and records.
In the wake of the "No" vote in the Scottish independence referendum, all parties participated in the Smith Commission, which recommended:
- Full control over income tax rates and bands, and air passenger duty
- A share of VAT should be assigned to the Scottish Parliament
- Control over certain types of benefit payments.
Backed by the three largest pro-Union parties, these changes formed the basis of legislation which has since come into force.
The SNP, which forms the Scottish government at Holyrood, maintains that these powers are still short of the mark and continues to argue for full independence.
The National Assembly for Wales sits at the Senedd in Cardiff Bay, and is made up of 60 elected Assembly Members (AMs). The Welsh government is its executive branch.
Since 2011 the Welsh assembly has had primary law-making powers in devolved areas, and in 2014 it gained tax-raising powers including stamp duty and landfill tax.
The Wales Act 2017 became law in January putting devolution in Wales on a similar footing to that in Scotland. The legislation devolves to Wales extra powers over transport, energy, electoral arrangements and, since April 2019, has included the ability to set income tax levels at a different level from England.
It also defines what remains under Westminster control, with everything else assumed to rest with the assembly - known as the "reserved powers" model.
The Assembly has control over public services such as health, education and local government. In addition, it can make laws relating to ancient monuments and historic buildings, public administration, sport and recreation, tourism, town and country planning, flood defences, the assembly itself, and the Welsh language.
The Northern Ireland Assembly sits at Stormont in Belfast, and is made up of 90 elected Members of the Legislative Assembly (MLAs).
The power-sharing agreement between the nationalist and unionist communities in Northern Ireland is critical to the functioning of the assembly; devolution of powers has been suspended and reinstated several times since it started in 1998.
The Assembly is currently without an executive after the power-sharing coalition broke down in March 2017. The political parties have now been in negotiations to restore the executive for over two years.
In July 2019, the UK Parliament passed the Northern Ireland Executive Formation Act to further extend the suspension of the assembly, and avoid having to call an election while the negotiations continue. This extension period will now end on 13 January 2020.
Devolution here is slightly different to Scotland and Wales, with government powers divided into three categories: transferred, reserved and excepted.
- Reserved powers - which could be transferred in the future with cross-community consent - include prisons and civil defence
- Excepted powers - including parliamentary and assembly elections, international relations and defence - cannot be transferred without primary legislation from Westminster
In addition to the main devolved powers, the assembly can also legislate on culture, arts and leisure, learning and employment and regional and social development.
As part of the Stormont House agreement, new tax powers were devolved to the Assembly, with the possibility of setting its own rate of corporation tax from April 2018. However, with the Northern Ireland executive suspended, the terms of the final agreement still await discussion.
As devolution to Scotland, Wales and Northern Ireland has gathered pace, many people have argued there is not enough devolution to and within England.
There are two particular sources of tension.
The Treasury gives block grants to Scotland, Wales and Northern Ireland to spend on devolved policy areas, calculated through the Barnett formula on the basis of population.
Scottish, Welsh and Northern Irish MPs' can vote on laws that only apply in England - which some people believe should be addressed through English votes on English laws, the idea that only English MPs should be able to vote on laws which predominantly affect England.
The parties at Westminster disagree over the best way to address the puzzle of which MPs get to vote on which laws, and have set out competing solutions.
On top of that, there have been renewed calls for devolution to England's cities and regions.