Each sets out a view of the world in a more than geographical sense. That now-familiar photo-image of our planet taken from the moon is as much an icon – Spaceship Earth – as any of the mappae.What makes maps interesting is that you can’t be wholly relatavistic about them. The arrangement is chronological, but the organisers are keen that we shouldn’t simply see a story about the progressive improvement of geographical and astronomical knowledge. Rather, every map is a conceptual scheme, an imaginary picture, a work of art and a cultural document.This is quite true: the history of maps incorporates the histories of fine-penmanship, semiotics, zoology, geometry, sea-faring, religion et al Mapping motives are always so mixed and so large. Very few of the world-plans here would be of much practical use to a traveller.

Not until the 19th-century, when every land-mass was at last reckoned, would there again be such confident images of the whole world.You can find several mappae displayed in “The Earth and the Heavens”, a British Library exhibition at the British Museum, which presents an extensive collection of world-maps and star-maps, ancient and modern, though predominantly Western. Conceptually, however, it knows everything: it admits no incompleteness. There are no terrae incognitae lurking at its borders, no half-charted coastlines trailing off into doubt, and (whether or not these map-makers were really flat- earthers) no awkwardness about projecting the round globe on to a flat surface. Though mappae mundi were an English speciality, information about the British Isles isn’t much more solid. Wales and Scotland often snuggle into the bottom left- hand corner – as islands.
Geographically, the mappa mundi knows very little. A circular world is bounded by a circular ocean, and divided into three parts by a T-shaped sea.

The top semicircle is Asia (east is always at the top), the bottom right section is Europe, and the bottom left Africa Jerusalem is set bang in the centre The T stands for, roughly, the Med. This neat geometry may be elaborated to include rivers and islands, and to represent coastlines with a little more accuracy The Red Sea is included, coloured bright scarlet Adam and Eve in Eden appear at the very top Weird humanoids are shown along the edge of Africa. Like many more recent design ideas, the medieval mappa mundi may not be very useful, but it’s certainly rational At its most basic, it hits what’s called a T-O format. There they can collect their Seasalter oysters and Veuve Clicquot champagne, supplied by The Balls Brothers wine bar stand, Hays Galleria, London Bridge City, Tooley St, London SE1.. There are six free oysters and half a bottle of champagne on offer for the first five readers to take a copy of this issue of the Independent to the Seasalter Shellfish oyster stand at the fair.

Apart from the wide selection of seafood and oysters to taste and buy, there will be live cajun and zydeco music from the American Deep South, a jazz band, and champagne and Pimms stands. To celebrate the start of the oyster season, the annual Hay’s Galleria Oyster and Seafood Fair will run from 1-3 September in the riverside setting of Hay’s Galleria, a beautiful converted wharf. For the grand gesture, place an ad to that effect in the classifieds, with the excuse that you are giving the money to charity.The Postcard Traders’ Fair runs from 30 August to 2 September at the Royal Horticultural Society Halls, Greycoat St, London, SWI (0171-834 4333). Were you in Paris, for example, you could send postcard reproductions of Courbet’s The Origins of the World – a 19th-century painting of a woman’s genitals which has recently gone on show at the Musee d’Orsay.”Sending them in envelopes, of course, also means you have more room to write in.

But if that subtly ironic observation of your host country continues to elude you, you could resort to buying your stamps in the lowest denomination available and fill the card up that way.Or you could just dispense with sending them altogether. Indeed, the finest postcard I ever sent was of the beauty spots of Hull, from the Hull Tourist Board. Alternatively, you can get postcards from the first-class section on Lufthansa, even if you are flying economy. Unfortunately, they are all of their airplanes, but they do also provide damn useful airmail envelopes.”He suggests always using envelopes, which the very organised can type before setting off. “This means you can describe scatological episodes and send wicked pictures that would otherwise be confiscated. Says James Bentley, a seasoned travel writer: “Go into hotels posher than the one you’re staying in – ideally somewhere really plush like the Imperial in Vienna – and ask the porter for some of theirs.”"Local tourist boards are an excellent source. Only three (estimated value pounds 2,500) are known still to exist, although hundreds were sent.