To become a London cabbie requires more than guidebooks and a smartphone. There is a test—some say the world’s toughest test—known as The Knowledge. In order to pass, a driver needs to navigate the city’s 25,000 streets without a map or help from GPS, and identify every possible tourist spot, no matter how obscure. Case in point: Great Newport Street’s Metropolitan Police coat hook.

The wrought iron hook hanging next to No. 4 goes back to the early days of automobile traffic. The building sits just off the corner of a chaotic six-street convergence, and even with the guidance of traffic lights drivers didn’t always trust the signals. So the police were assigned to step in now and then, to keep things moving, and if it happened to be a hot summer day they needed a place to hang their heavy woolen coats. Since No. 4 was under construction there was a handy nail to do the trick, but once construction was completed, the nail disappeared.

The makeshift hook may have been gone, but the traffic wasn’t, so the police asked for the nail to be put back. They got this instead: a sturdy bespoke model, clearly labeled so everyone knew who it was for. There have been subsequent renovations to the little building, but the coat hook is still there, still challenging cab drivers’ “knowledge” to find it.