Why does Spain’s Article 155 exist, and what does it actually say?

On Friday, the Spanish Senate invoked Article 155 against Catalonia, a region in the country's northeast. Just minutes before, Catalonia had voted to declare independence from Spain. In response, Madrid invoked something that's been described, alternately, as a “nuclear option” or “atomic bomb.” Article 155, which spans just two short paragraphs of the 1978 Constitution […]

Source: http://www.washingtonpost.com/blogs/worldviews/wp/2017/10/27/why-does-spains-article-155-exist-and-what-does-it-actually-say/