A self-confessed "grammar vigilante" has been secretly correcting bad punctuation on street signs and shop fronts for more than a decade.
The anonymous crusader carries out his work in the dead of night using the "Apostrophiser" - a long-handled tool he created to reach the highest signs.
The man, who wishes to remain anonymous, told the BBC that correcting rogue apostrophes is his speciality.
He uses stickers, rather than paint, to cover up errors.
"I do think it is a cause worth pursuing. I have felt extremely nervous and the heart has been thumping," he told the BBC.
But the man maintained that he had not committed any crime.
"It's more a of a crime to have the apostrophes wrong in the first place," he said.
The vigilante, who works in engineering, uses sticky-backed plastic to aid his campaign.
"The first one I did was way back in 2003. It was a council sign, 'open Monday's to Friday's', and it had these ridiculous apostrophes," he told BBC Radio 4.
"There was the really gross sign in Gloucester Road, 'Amy's nail's' - it was just so loud and in our face.
"I just couldn't abide it and had to go and apply my wares to this apostrophe.
"It grates. I'm a grammar vigilante."
He defended his actions, saying: "What I'm doing is sticking on a bit of sticky-backed plastic.
"It is a worse crime to have all these errant apostrophes on shops and garages and I just think it is going to teach the youth of tomorrow the wrong grammar."
:: 'The Apostrophiser' features on BBC Radio Four at 8pm on Monday April 3.