The power of the gesture is extraordinary. Hold up two fingers in most parts of the world and people will think you're celebrating peace or victory, or simply wanting two more beers. Do the same thing (palm inwards) in the UK, Australia or Ireland and you'll be halfway to a fight.
Here at home, the "thumbs-up" is the most benign of signals. Do the same thing in a traffic jam in Sardinia and you may find yourself run off the road. "Sit on this!" is what it signifies there. In Iran it's so rude there's a name for it: the bilakh.
In a world where, despite the best efforts of environmentalists, travel is on the up and up, getting these things right is ever more crucial. As TV images are routinely beamed around the world, the same applies. When President Bush won his second term back in 2004, he made a sign at his supporters known locally as the "hook 'em, Horns". This has the index and little finger outstretched while the two middle fingers are curled back into the palm. For stalwarts of the Texas Longhorns football team, this means "victory". Poor George. He can hardly have known that, in Italy, it implies that a man is a cuckold - while in Norway it's the sign of the devil.
It's not just the deliberate gestures we have to watch out for as we try to negotiate our ever-shrinking world. Even ordinary movements can cause problems. Fold your arms in front of your chest at home and people will think you're relaxed and at ease; do the same in Finland and you'll be seen as rude and stand-offish. Cross your legs on the Underground in London and nobody will turn a hair; but if in doing the same at Doha International Airport you show someone the "unclean" sole of your foot, you'll be insulting them. When Iraq fell in 2003, people vented their anger on their tyrannical ex-president by beating his effigy with the soles of their shoes.
Pointing is another gesture we all use over here without compunction. In such places as Malaysia or West Africa, however, pointing with a finger is one of the rudest things you can do. Malaysians point with a closed fist, the thumb at the top indicating direction. Filipinos are even more low-key, singling out an object by shifting their eyes towards it and pursing their lips in its direction. In Guinea-Bissau people take this a step further, gesturing with chin or stuck-out tongue.
Globalisation is changing all this, of course. In Japan, people point and catch each other's eye in a way they never used to; new Russians look on the old strictures of nyekulturny (uncultured behaviour) with a chuckle; and young Iranian women wear clubbing gear under their chadors. In a generation's time, the globe may be the equivalent of the contemporary British high street: grimly faceless and uniform. So, if you want to cause outrage in Turkey with the American "OK" sign, get out there and do it now.
Mark McCrum's "Going Dutch in Beijing: the International Guide to Doing the Right Thing" is published in October by Profile Books