On the Underground, my family is entranced by the phrase ‘Mind the gap’. Mind in the sense of ‘to remember’ or ‘bring to mind’ is practically obsolete, surviving only in expressions such as ‘Mind your manners’. To children and foreigners, gap most commonly suggests an American clothes shop. I explain that the formula can only have been devised during the Modernist period, when all extraneous ornament was pared away. Just as the Underground map distils the freaks of London’s geography into an elegantly comprehensible pattern of coloured lines, so ‘Mind the gap’ reduces language to its smallest atomic parts. Unfortunately, it is meaningless to people who don’t already know. But don’t change it.

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Language, like townscape, is enriched by the historic survivals embedded in otherwise banal thoroughfares. I must thank Judy Parkinson’s new book Spilling the Beans on the Cat’s Pyjamas for decoding some of them. ‘Pearls before swine’, ‘cut the mustard’, ‘pass the buck’, ‘running the gauntlet’-oh, read the book! Otherwise random colours resolve themselves into meaning, with the satisfaction that I’m told comes from finishing a Rubik’s cube. My children will find that I now deconstruct these linguistic time-capsules, much as I point out the statues that we pass in the car. It will be another thing that makes their father so irritating.