Only God really knows if Richard Dawkins had those Keep Calm And Carry On posters in mind in the mid-1970s when he coined the word "meme", meaning "an idea, behaviour or style that spreads from person to person within a culture."

But if he didn't, I can't think of a better illustration of the concept.

Originally intended for a second-world-war poster, the phrase was redeployed a decade ago and, meme-like, spread from person to person and subject to subject. It has been subverted and parodied but without losing its essence. I've yet to see Keep Calm And Curry On but if it's true John le Carre has a poster in his downstairs loo saying Keep Calm And Le Carre On, then it's infiltrated just about every aspect of culture.

That means it's also on T-shirts. As I write this, the Edinburgh Festival is in full swing which, if you believe the hype, means every street corner is occupied by a Bosnian stilt-walker juggling chainsaws and telling lightbulb jokes. The hype is wrong.

Every street corner, and most stretches of pavement in between, is actually occupied by tourists looking for Bosnian stilt-walkers to Tweet pictures of, and a great number of said tourists are wearing Keep Calm And Carry On T-shirts, or some variation on them.

My favourite so far is Keep Calm It Wisnae Me, spotted on Princes Street. My least favourite is I Can't Keep Calm Because I'm Spanish. I'm not sure if there was a backprint saying I Can't Keep Gibraltar Either For The Same Reason, but if there wasn't there should have been.

Look around the shops selling See You Jimmy hats and yellow plastic ponchos and you realise the permutations on the theme are endless. The legends available range from the obvious (Keep Calm And Have A Beer) to the geeky (Keep Calm And Read Harry Potter) to the political (Keep Calm And Vote Yes) to the downright weird (Keep Calm And Love A Tattooed Nurse). The wordsmiths among you might like Keep Calm And Carrion, which bears an image of a crow, or Keep Calm And Use An Affirmation.

There is even (oh dear) a Fringe show using the concept. It's called Keep Calm And Listen To Classic, though classic what isn't stated. For the record, it's on at St Cuthbert's Parish Church at the corner of Princes Street and Lothian Road and consists of South Korean folk songs performed by the Korea Union Youth Chamber Orchestra and Choir. Entrance is free, which isn't surprising really.

So as you navigate the crowds this festival, keep an eye out for Keep Calm T-shirts. You can even play Keep Calm T-shirt bingo: every time you see one, find a pub and raise a glass. Better still, find a Bosnian stilt-walker ­juggling chainsaws and ask him how many atheists it takes to change a lightbulb. Keep calm and I'll give you the punchline next week.