Spain has approved direct rule in Catalonia after the Catalan parliament declared independence.

The move gives the government of Mariano Rajoy to trigger Article 155 and rule Catalonia directly.

Article 155 has often been described as the "nuclear option" and has been rolled out amid a struggle between Barcelona and Madrid following the Catalan referendum on October 1. But what is Article 155?

Article 155 is only two short paragraphs of the 1978 Constitution of Spain.

It says that if a regional government "doesn't comply with the obligations of the Constitution or other laws it imposes, or acts in a way that seriously undermines the interests of Spain", the national government can ask the Senate to vote on the use of the measure.

The Constitution states that an absolute majority must approve the article's use.

If this happens, the national government may adopt the "necessary methods" to force a regional government to comply in order to protect "said interests".

The only directions given state that the national government may give orders to "all authorities" of regional governments.

Josep Costa, a professor of political science at Barcelona's Universitat Pompeu Fabra, told Al Jazeera: "The Spanish government really didn't have a plan to proceed with 155, so there's a lot of uncertainty as to what can be done and what can't."

Costa claims he has seen many proposals, such as the creation of an interim government to run Catalonia, that are "clearly not possible" under 155.

"There's no doubt [Article 155] has limits … including the Catalan statute of autonomy, which cannot be repealed," Costa said.

However, the Spanish government said on Thursday that it was confident triggering the article would help Madrid restore "legality and constitutional order".