As well as being a member of the European Union (EU) - a union of 28 countries with integrated policies on issues such as justice, defence and industry - the UK is a member of the European Economic Area (EEA), a single market that allows for the free movement of people, goods, services and capital.
The article 127 court case is at root of whether the UK can leave the EU but stay in the EEA.
What is article 127?
The EEA Agreement provides for the free movement of persons, goods, services and capital within the EU single market and extends the single market to non-EU states Norway, Iceland and Liechtenstein.
Article 127 sets out the mechanism for leaving the EEA, in the same way that article 50 of the Lisbon treaty sets out the steps that a country must take to leave the EU.
Article 127 says that each 'contracting party [country] may withdraw from this agreement provided it gives at least twelve months' notice in writing to the other contracting parties.'
The court case
The claimants who brought the article 127 court case say that the UK is subject to two separate membership agreements, one for the EU, the other for the EEA. As each agreement has its own exit procedures a separate decision has to be taken on leaving the EEA, independent on the decision to leave the EU.
Just as the Supreme Court has said that parliament must vote to trigger article 50 to leave the EU, the High Court was asked to declare that parliament must take a vote to trigger article 127.
That's why we said earlier that the issue at stake in this case is whether the UK can leave the EU but stay in the EEA.
The government case is that as the UK is party to the EEA agreement only in its capacity as a member of the EU, once the UK leaves the EU it would automatically cease to be a member of the EEA. The claimants' argument is that as article 127 provides an express mechanism for withdrawal, it implicitly excludes other ways of leaving the EEA, such as ceasing to be a member of the EU.
The court has found in the government's favour, but at the time of writing the full decision was yet to be published.