The residents of London have an advantage over the rest of the population: they can define their location, and hence their degree of devotion to the city, according to a series of concentric circles. Your telephone dialling code or Travelcard zone says as much about you as your postal address ever can. And the closer in you get, the nearer you become to the genuinely good life of the true city-dweller.
Thanks to the M25, 10 years old next Tuesday, the urbanite has a simple quest: to live within London's ring road, as close to the hub as possible. The birthday of London's ring road is a good moment to assess the magnetism of the capital. You live either inside the M25, or outside it - nothing, evidently, can exist actually on it. If you want something delivered free of charge, or need your mobile phone to work without a flicker, then it is convenient to reside within the concrete circle.
Next along is the actual boundary of Greater London. Frankly, this is not much cop in terms of defining whether or not you are a Londoner. Leafy fingers of the London Borough of Bromley extend deep into the Garden of England, the residents having much more in common with Kentish farmers than Deptford factory workers. Yet Watford, that tangle of shopping centres, slip roads and housing estates that epitomises suburbia, is technically outside Greater London. This may save on insurance premiums, but it adds to the cost of travel.
Fares on London's buses and tubes are computed according to concentric zones established before the GLC was taken out and shot for opposing the bulldozer of Thatcherism. Fifteen years ago, the revolutionary idea that fares could be charged in terms of broad bands rather than hundreds of individual calculations took root. There has been a bit of tinkering with the boundaries since then, but it remains the case that an annual season ticket from Aldgate East is pounds 112 cheaper than one from West Kensington.
There is no difference in telephone costs between 0181 and 0171, but it could affect the value of your property. The telephonically bizarre idea of differentiating between the central core and the outer ring of London (rather than, say, the north and south of the river) was introduced in 1991, and instantly established another layer of snobbery to the capital.
"Definitely the code makes a difference - 0171 is more exclusive," says Alan Lowne of the estate agent Bairstow Eves in Harringay (0181 prefix, N4 postcode). "But what's more important is the postcode. Some of the streets around here are on the border between N15 and N4, and if it's N4 it puts quite a few thousand onto the price."
Not a lot of Londoners know the way in which London's postcodes are organised. The most central district gets a "1" allocated (W1, N1, etc), but after that numbers are allocated according to alphabetical order. So SW2 is Brixton, SW3 Chelsea, SW4 Clapham, etc - bouncing randomly around London. Clear winner in this peculiar system is Chingford; Norman Tebbit's constituency is officially London E4, which you might think places it just an eel's throw past Whitechapel but in reality turns out to be halfway to Harlow.
Stray just a mite too far north and you find yourself in EN9 - that residential no-man's-land just within the M25 but outside Greater London, once part of Middlesex but now on the cusp of Essex and Hertfordshire.
The prospect makes me shiver. Tonight the clocks go back, and as winter settles in with a vengeance we people deep in the city will be a good few degrees warmer than those on the fringes, because central London really is where it's hot. Even if the whole transport system freezes over, we centrists couldn't much care: if you can't walk to it, goes our motto, it must be in the suburbs - and therefore by definition irrelevant.
From my front door (postcode prefix 1, LT zone 1), I can march on Parliament or stroll to Covent Garden in 15 minutes. The local cinema is the National Film Theatre, and what passes for a community arts centre is the Young Vic. One tiny problem: the first two letters of my postcode are SE, which means I am outside that most coveted inner sanctum of all - the Circle Line. The underground loop sticks determinedly north of the Thames. And they're such insufferable snobs up there.