Secret codes aren't just the preserve of spies. There are mysterious symbols, numbers and dials all around us which mean, or have meant, something to someone at some stage. From 19th century insurance policies to markings for homeless people, covert communication between Londoners past and present are etched all around us.
We set about uncovering the mysteries behind some of these often familiar, but seldom deciphered, hidden codes.
Mystery right under our feet
When you're next strolling around town, keep your eyes peeled for these curious symbols carved into Victorian paving stones, from carefully crafted Maltese crosses to shoddily carved-out letters. Once you’ve noticed one, you’ll start to see them everywhere.
It appears that, despite their prevalence, no one has satisfactorily uncovered the mystery of what these symbols actually represent. We've had our fair share of speculation, of course, from conspiratorial conjecture that they directed Freemason initiates to hidden lodges, to more mundane theories that they literally just identify the location of electricity, gas and water.
The mystery continues.
H is for hydrant
Yellow H signs are everywhere, and serve a very useful purpose; they point the fire services in the direction of the nearest hydrant. The top number indicates the size of the water main, and the bottom number indicates how far the hydrant is from the sign.
You'll notice that modern signs show measurements in millimetres and metres and older signs show them in inches and feet. Next time you come across one, see if you can locate the nearby hidden hydrant.
19th century fire insurance policies
Imagine being trapped in a burning building, while being completely ignored by the fire service because you don’t have private insurance. Sounds like the plot from a dystopian film, doesn't it? Well, this was actually the case back in the 18th and 19th centuries.
Back then, there was no public fire brigade, so Londoners would need to insure against fires by paying private fire-fighting teams. People would fix badges to the front of their houses displaying the relevant company logo to prove the insurance was paid.
If you're lucky, you can still spot these plaques on the front of houses around London. We found this one in Goodwin's Court near Leicester Square (which is incidentally where a scene from Harry Potter was shot).
Hobo codes originated in the 1930s in America during the great depression. Because of job shortages, those looking for work would travel from place to place, and these chalk markings would indicate which dwelling places were safest to sleep in. But in 2007 The Pavement magazine created their own code to enable homeless people around London to share their experiences.
Val Stevenson, web manager at the Pavement, said: "Look for symbols at knee rather than eye height. I’ve spotted the hobo codes all around London, and recently in Liverpool Street."
How do you find a lamp post?
Lamp posts come in all manner of shapes and sizes, old and new, but have you noticed that they all have a unique number?
These numbers act as identifier for your local council and make operation and maintenance a lot easier. Councils also give asset numbers to bollards, traffic signals, parking signs and meters.
What lies beneath
It may look like the work of an overzealous street artist, but if you come across colourful squiggles, dots and arrows painted onto the pavement, it’s more likely to be the secret coding system for our utility companies.
In London, more than 50 different utility companies dig up the streets so it's important to have a universal coding system to indicate what’s hidden underground. Every colour has a meaning; red points to electrical lines, blue identifies water pipes, yellow means gas, green is CCTV networks and cable TV lines and white is just general communication, from measurements to instructions.
Sensors on the tube
Leigh Dodds, self-professed 'data magpie' from the Open Data Institute, drew our attention to the mysterious dials on tube trains. He said: "Have you noticed the dials on underground carriages? These are brake cylinder gauges which indicate air pressure in real time. These are used by operational and maintenance staff for fault-finding."
The cylinders are in the passenger compartment to allow the driver to access them in case of a brake defect or dragging brake.
Hidden underground directions
Have you see these blue signs on the underground? We’ve already revealed these codes but they deserve an inclusion here too.
These signs are called Station Identification (SID) numbers. They exist to make it easier for the rescuers to navigate the tube network when visibility is poor (i.e. in smoky conditions) and are also used by station staff and contractors to identify faults in particular rooms.
The top number denotes the level below ground, while the remaining numbers define a location on that level. So, now you know.