Trafalgar Square is a notoriously busy spot, one that is packed with tourists crowding around its famous statues and intriguing attractions. But many of these visitors don’t know that there are pieces of London’s past hidden right under their feet.

A series of plaques are embedded within the northeastern part of Trafalgar Square. Turn your eyes to the wall and the stairs leading toward the National Gallery, and you’ll see various official Imperial measurements—some more familiar than others—that have lain in the square since 1876.

The plaques depicting inches, feet, and yards as measured at 62 degrees Fahrenheit do more than give tourists a crash course on Imperial units (even those who favor the Imperial system may still be perplexed by the plaques for chains, perches, and poles). The Board of Trade installed these markers to preserve the standard units of measurement that were created between 1838 and 1842.

A standards commission overseen by Sir George Airy created three copies of the official units of measurement after a devastating fire in the Palace of Westminster destroyed the old ones. Trafalgar Square isn’t the only place within London to spot these official measurements. You can also find some in the Great Hall of the Guildhall and near the Royal Observatory in Greenwich.