Article 155 of the Spanish constitution would allow the government in Madrid to intervene in the running of Catalonia. It’s an extreme measure for exceptional situations that has never been invoked before.

Article 155 is the most extreme measure available to Spain’s government, allowing it to take over the running of an autonomous region, should it declare independence. That could be invoked as soon as Saturday.

In political circles, the article is known as “the nuclear option.” Former Foreign Secretary Jose Manuel Garcia described it as “an atomic bomb.”

The national legislature describes the article as an “exceptional or extreme” measure “for situations that are equally exceptional or extreme.”

The article allows “all measures necessary to compel the community to meet said obligations, or to protect the above-mentioned general interest.”

The measure has never been invoked in Spain. Similar provisions in other European countries have also never been invoked.