Flickr/House of Commons, CC BY-NC-NDAndy Burnham’s call for a codified UK constitution has revived debate about how the UK is governed. It would be a huge departure for the UK, and a major undertaking. But beyond its historic significance, there is another question: how do you make sure the population understands what the constitution does and who it’s for?A constitution sets out the rules for how a country is governed; who holds political power, what limits that power and the relationship between parliament, government and the courts. Countries such as the US and Germany have a single constitutional document. But in the UK, the rules and principles have developed over centuries. These include parliamentary sovereignty, rule of law, judicial independence and devolution, which determine who holds power and how it is controlled. These rules are not in one document but come from different sources over the years, including acts of parliament, court decisions and constitutional practices. This is why those in favour of changing this often call for a “written” constitution – though it’s more accurate to say that it is “uncodified”. However, putting such complex principles into a single document would not in itself make them easier to understand.Supporters of a codified constitution argue that it could make the rules clearer and strengthen accountability. For the prime minister, it could also provide an opportunity to clarify where power lies as more of it is devolved across the UK, setting out more clearly the powers held by central government and those exercised by devolved governments and local authorities. Critics point to the flexibility of the current system, which allows the UK constitution to change over time without amending a single document.Public understanding matters. People cannot take part in meaningful debate about constitutional reform if they do not understand the complex ideas they are being asked to consider. The challenge of a written constitutionMy ongoing research examines what happens when constitutional ideas move from specialist legal settings into public debate. Brexit is a clear example of this.Constitutional ideas do not move unchanged from legal doctrine into everyday political discussion. They must be explained in ways that make sense outside courts, universities and parliament. The language used shapes how those ideas are understood. Writing a constitutional document will involve more parties than just parliament. Mistervlad/Shutterstock For example, back in the late 19th century, a book on the UK constitution set out what became the traditional principle that parliament has the authority to make or unmake any law, and that this authority cannot be challenged in court. This was the principle of parliamentary sovereignty. Before the 2016 EU referendum (more commonly known as Brexit), it was a concept only really discussed by constitutional lawyers, academics, judges and politicians.But with the referendum debate came the phrase “take back control”. It showed how language can shape the meaning of a constitutional idea in public debate. These three easily repeatable words helped to communicate a complicated set of constitutional relationships. Sovereignty became part of a simpler story: control had been lost and could be taken back.Making complex constitutional ideas easier to understand does not mean getting them wrong. Few people can be expected to study centuries of constitutional law before taking part in something like the EU referendum. But the words used to explain those ideas matter: they can shape public understanding and democratic engagement.Explaining these ideas means making choices. Some issues will receive more attention than others. For example, questions about where political power should lie may be emphasised over questions about how that power should be controlled. Abstract ideas such as parliamentary sovereignty need examples. Complex relationships between institutions, such as parliament and the courts, also need to be explained in language that makes sense outside legal and political circles.Different political voices present the choices differently. Burnham links a written constitution to devolution and where power lies, while Liberal Democrat leader Ed Davey proposes it to protect fundamental rights.This is why communication matters. Public debate does more than pass legal knowledge from experts to the wider population. The way constitutional principles are explained can affect what those principles come to mean in public debate.None of this is an argument for or against the UK having a written constitution. But there is a question over whether an uncodified constitution necessarily provides fewer protections than a codified one. In my published work, co-authored with constitutional law expert John McGarry, we found that an uncodified constitution can still provide important protections for constitutional rights and values.The choice is not simply between constitutional protection and no protection. The UK system already provides some protections associated with a written constitution, while also allowing the constitution to change over time.Burnham’s proposal brings us back to the question at the heart of this debate. Before asking the public whether the UK needs a written constitution, we may first need to get better at explaining the constitution that we already have.Sharon McAvoy does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.