A pair of anonymous vigilantes are cleaning up Quito’s graffiti; by adding accents, inserting commas and placing question marks on sentences scrawled across city walls

In the dead of night, two men steal through the streets of Quito armed with spray cans and a zeal for reform. They are not political activists or revolutionaries: they are radical grammar pedants on a mission to correctly punctuate Ecuador’s graffiti.

Adding accents, inserting commas and placing question marks at the beginning and end of interrogative sentences scrawled on the city’s walls, the vigilante editors have intervened repeatedly over the past three months to expose the orthographic shortcomings of would-be poets, forlorn lovers and anti-government campaigners.

The first images of this guerrilla nitpicking exploded across social networks in December, but despite their global notoriety, the group –Acción Ortográfica Quito (or Quito Orthographic Action in English) – have kept their identities secret and had never given an interview to a newspaper until they spoke to the Guardian.

“We call what we do orthographic vandalism,” one of the members, who only gives his nickname, Diéresis, said. “Graffiti is an act of vandalism. By correcting it, we turn it into something ironic.”

Facebook Twitter Pinterest Diéresis says he was goaded into activism by an appallingly punctuated piece of graffiti on a wall he frequently passed by. Photograph: Acción Ortográfica

Graffiti is an act of vandalism. By correcting it, we turn it into something ironic. Diéresis

Diéresis – a lawyer in his thirties – says he was goaded into activism by an appallingly punctuated piece of graffiti on a wall he frequently passed by.

“I couldn’t believe that in just two sentences there were more than 10 grammatical errors,” he said over a cappuccino spiked with amaretto. So he grabbed a discarded pizza box, cut it into a stencil, called on a friend to help and then got to work on the garbled text.

In went two question marks, two accents, three commas, a dot over an “i” and a space between “por” and “que”, out went an ellipsis and down went a wrongly capitalised “P”. The end result: “¿Para qué y por qué, mi amor? Por ti, por mí, lo siento.” (“For what and why, my love? Because of you, because of me, I’m sorry.”) – now conveys emotional heartbreak instead of creating semantic headaches.

“Grammatical errors cause stress. We only make texts comprehensible that otherwise would not send any message whatsoever,” says Diéresis – but he insists that the main aim is entertainment not education. “While we are promoting the correct use of language, it is also an excuse for a bit of fun. The idea of making a passerby smile is rewarding.”

Facebook Twitter Pinterest ‘I never thought that our action would have that kind of consequence,’ Diéresis recalls. Photograph: Acción Ortográfica

The image of the red corrections on the black slogan went viral on Facebook and Twitter. “I got the news by chance after someone in Argentina shared a photo with a friend in Scotland who shared it with a mutual acquaintance. I never thought that our action would have that kind of consequence,” he recalls. Copycat vigilantes have since amended writings on the wall in Madrid.

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Buoyed by their celebrity, Quito Orthographic Action have taken on a third member for online work. All take their names from punctuation marks: as well as Diéresis (who is named after an umlaut-like symbol), there is Tilde (an accent) and Coma (comma).

The interventions are carefully planned. During the day, the group scour the city for egregious misspellings, heinous orthographic errors and irritating omissions. The targets are photographed, the team debate what the original authors intended to say, and then they move in late at night to set the written word to rights.

They are unsure if their work breaks the law. Police have seen them at work and taken no action. But they choose to remain anonymous because they are concerned about possible political repercussions. Ecuador’s President Rafael Correa has used his regular TV addresses to attack those who mock him and called on his supporters to strike back.

“This is a hypersensitive moment in Ecuador,” Diéresis says. “On national television, the president has revealed the personal information of Twitter users who criticise or make fun of him. That is low, unethical and illegal and it gives you a good picture of what’s going on in this country. It also got me thinking, ‘what would they do to me?’ In situations like this you have to be creative”.

Facebook Twitter Pinterest ‘This is a hypersensitive moment in Ecuador,’ Diéresis says. Photograph: Acción Ortográfica

His fears may have been exaggerated if the group had stuck to graffiti, but the newest member of the group, Coma opened a Twitter account under the name “Acción Ortográfica Quito” and recently corrected a tweet from Correa – including missing accents, commas and question marks.

“Some people made me promise that we would not attempt to correct Correa and I agreed,” Diéresis says. “It seemed silly to correct someone with such big issues. But then Coma intervened with Correa’s tweet. That day, whenever my phone buzzed, I freaked out.”

To maintain their political impartiality, the group also corrected a tweet from the office of the Quito mayor, who is one of Correa’s main political opponents.

Despite such concerns, however, they have so far operated without problems, though, inevitably, when it comes to grammar not everyone is happy, especially those whose mistakes have become a subject of public ridicule.

“The only person really annoyed by our actions is the author of the first graffiti we corrected,” said Diéresis. “We know this because we have friends in common.”

• This article was amended on 19 March 2015. An earlier version said Acción Ortográfica Quito had never given a media interview until they spoke to the Guardian. The group had suggested that that was the case, but they have since clarified that they gave an earlier interview to Colors magazine, although the Guardian was the first media organisation to contact them.