The article grants Madrid the right to "give orders" to "all authorities" of a regional government if said 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".
And so, today, as I stood with several thousand Catalans outside their parliament, and as national police helicopters circled overhead, regional legislators voted on a formal and unilateral declaration of independence.
Catalonia, a region of 7.5 million people in the region bordering France, generates about 20 percent of Spain's economic output.
However, the Welsh and Scottish governments both backed the Catalans.
The resolution approved by lawmakers in Barcelona said the establishment of Europe's newest sovereign country had been set in motion.
Jews of Catalonia, an anti-secessionist group, on Friday lauded the Madrid-based federation for its "important" message, as they called it on Twitter.
"Canada has recognized many-many countries since the creation of the United Nations and there is really no reason for not recognizing the new Catalonia country, which will become the 194 country of the United Nations", Ouellet told CBC's Power & Politics program. "Kol HaKavod FCJE! The separatists represent only themselves".
Catalans are divided on the question of independence - an opinion poll earlier this year said 41% were in favour and 49% were opposed to independence.
"We welcome the birth of our Catalonian Republic".
Article 155, which spans just two short paragraphs of the 1978 Spanish constitution, is 84 words long.
Within an hour, the upper house of Spain's parliament in Madrid authorized [Spanish prime minister Mariano] Rajoy's government to rule Catalonia directly.
Puigdemont refuses to say whether he had declared independence and instead calls for dialogue.
Banco Sabadell, Catalonia's second largest bank, announces it will shift its registered domicile out of the region. "The imposition of direct rule cannot be the solution and should be of concern to democrats everywhere". But Catalans have been divided on the question of independence.
Regional elections in 2015 returned a slim majority of pro-independence lawmakers, who took this as a mandate to push ahead with the independence drive.
The same crowds that cheered each Yes vote from Catalan MPs reportedly booed Mr Rajoy as he made his announcement. "Spain remains our only interlocutor".
"These are sad and anxious times", Rajoy said, adding that Friday's vote "ended the Catalan tradition of fostering agreement".
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