The imperial measures of Trafalgar Square were installed in 1876, detailing the official measurements of inches, feet, yards, links, chains, perches and poles.
A standards commission, chaired by Sir George Airy (a mathematician and astronomer), created new standard units of measurement between 1838 and 1842, making three copies of these measurements across London.
As well as in Trafalgar Square, the standards can be found in the Great Hall of the Guildhall and outside the Royal Observatory in Greenwich.
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