From here:
There cannot be anyone in the civilised world who does not already know the basic rules of Mornington Crescent, so we shall not insult our readers by re-iterating them here. Suffice to say, if you have temporarily forgotten them, or if you come from, say, the uncivilised world, such as, for example, France, you will certainly pick them up as you go along. Beginners will discover that Mornington Crescent is a little like golf, a little like shove-ha’penny, quite a lot like watching your laundry in the tumble-drier, and most closely resembles feeling around in the dark for a pocketful of loose change dropped in an unlit, damp alleyway on a Saturday night after a few beers. That is to say: frustrating, hard on the right forefinger, disorienting, even more disorienting, sheer hell on the right forefinger, and frustrating… probably in that order.