Mornington Crescent is a deliberately obscure game introduced by the BBC Radio 4 programme I'm Sorry I Haven't A Clue, in part as a parody of chess commentary. It is named after Mornington Crescent tube station.
Players successively name streets in Greater London or railway stations on the London Underground network. No player is disqualified and the first player to say "Mornington Crescent" wins. Although no player is disqualified, repeatedly saying the same station is disallowed, and players cannot move if they are in 'knip'.
During play, the panellists often invoke obscure but authoritative-sounding rules and names of gambits. These "rules" are randomly conceived and the more ludicrous sounding, the better. For example "we're stuck in a Dollis Hill loop", or "once Tooting Bec has been declared, this move is not allowed unless two or more players are in Knip."
The chair Humphrey Lyttelton often introduces variants and alternative rules, for example the French version ("Chateau d'Eau"), or the King Edward Rules. For a list see this I'm Sorry I Haven't a Clue page
The game enjoys (or, possibly, suffers from) a culture of secrecy. Those in the know about the game enjoy pretending to others that all the rules are real, and that they really are in a rule book. Of course the rules aren't real - they are always made up on the spot (perhaps the only real rule is that panellists should be funny) and so the rulebook itself remains eternally elusive (or as Lyletton has put it on the show "[the rulebook is maintained with] inimitable accuracy by the lovely Samantha, who sleeps with it under her pillow. As it now runs to 17 volumes, she is running out of pillows".
Two books of 'rules' and history have been published, Stovold's Mornington Crescent Almanac by Graeme Garden, and The Little Book of Mornington Crescent by Graeme Garden, Tim Brooke-Taylor, Barry Cryer and Humphrey Lyttelton.
Seasoned players encourage new-comers to watch a game for a few moments and they'll 'soon pick up the rules as you go along'. In an excessive form of this secrecy BBC Radio Four once broadcast the first of a 'two part documentary' on Mornington Crescent, which gave a history of the game through the ages. The promised second part, which would give an in-depth explanation of the rules, was naturally never broadcast.
Another part of the culture surrounding the game is the correspondence it generates from listeners. Letters from (the fictional) Mrs. Trellis of North Wales, who writes to express her bafflement by the game, are read out on the show.
Science fiction writer Michael Moorcock included a reference to the game in a comic book which he scripted, entitled Michael Moorcock's Multiverse. Since the comic was published in the US, the reference was clearly an in-joke for any British readers who happened to get hold of an imported copy.
In the 1980s postal gaming hobbyists invented a variant of Mornington Crescent for postal play. This is called Finchley Central and is essentially the same as the original except that 1) the target has been changed to Finchley Central for copyright reasons and 2) huffing is obviously impossible.