Nexthing Fun

The objective was to give the appearance of a game of skill and strategy, with complex and long-winded rules and strategies, to parody games in which similarly circuitous systems have evolved. In general, Humphrey Lyttelton (host/chairman of the program) would describe special rules to apply to that session, such as "Trumpington's Variations" or "Tudor Court Rules", so that almost every episode featuring Mornington Crescent introduced a variant.

There have been many variations. In one of them, first introduced in North Yorkshire, a player whose movement is blocked is considered to be "in Nidd" and is forced to remain in place for the next three moves. This tends to block the other players, putting them into Nidd as well and causing a roadblock. In one episode, every player ended up in Nidd and the supposed rule had to be suspended so that the round could continue.

Over time, the destinations named by the panellists expanded beyond the Underground. ISIHAC is recorded around the United Kingdom, and the game is occasionally modified accordingly. There have been versions in Slough and Leeds, as well as one in Scotland, played during the Edinburgh Fringe arts festival (where the name was changed to "Morningside Crescent"). In one episode, recorded in Luton, panellists named locations as far afield as the Place de l'Étoile in Paris, Nevsky Prospekt in St. Petersburg, and Pennsylvania Avenue in Washington, DC. However, a move to Luton High Street was ruled invalid for being too remote. In other episodes, an attempt was supposedly made to expand the territory to Manhattan (via Heathrow and JFK) but there was some disagreement as to whether or not the New York City Subway system was suited to the game. References have been made in various episodes of ISIHAC to international versions of the game, including "Mornington Croissant", supposedly based on the Paris Metro, and "Mornington Peninsula", the Australian variant. At least one full game of Mornington Croissant was played on air.

Lyttelton joked that the game predated the London Underground. "Tudor Court Rules" were described as "A version of the game formally adopted by Henry VIII and played by Shakespeare. At this time, the underground was far smaller than at present, and so the playing area also was more restricted, primarily due to plague."

Those who asked for the rules were told "NF Stovold’s Mornington Crescent: Rules and Origins" was out of print. They were also advised that "your local bookshop might have a copy of The Little Book of Mornington Crescent by Tim, Graeme, Barry and Humph."

A regular feature that introduces Mornington Crescent, is a fictional letters section which begins with the chairman's comments ("I notice from the sheer weight of this week's postbag, we've received a little over no letters" and "I see from the number of letters raining down on us this week that the Scrabble factory has exploded again"). The single letter each week is from "A Mrs Trellis of North Wales", whose incoherent letters usually mistake the chairman for another Radio 4 presenter or media personality. "Dear Libby" (she writes), "why oh why ... very nearly spells YOYO", or "Dear Mr Titchmarsh, never let them tell you that size isn't important. My aunt told me that, but then all my new wallpaper fell off."