If you're the sort of person who keeps their eyes open while walking around, you may have noticed these strange hieroglyphs beside London's bridges.
This is the Bridge Mark, symbol of the Bridge House Estates. You'll find the mark carved into stone, inscribed on plaques and even flying on flags on Tower Bridge.
The Bridge House Estates is one of the oldest organisations in the world. It was formed in 1282 by the City of London, charged with maintaining the medieval London Bridge.
Over the centuries, it acquired substantial wealth through tolls, rents and donations. The money was used to build up a property portfolio, which brought in yet more funds.
These swollen coffers were then used to build other bridges, including Blackfriars and Tower Bridge, and to acquire Southwark Bridge, which had been built by other means. The funds also allowed improvements to existing bridges. In 1909, for example, Blackfriars Bridge was widened to carry tram lines, all funded by the Bridge House Estates.
The Bridge House Estates continues to look after the City bridges today. Most recently, it stumped up most of the funds for the Millennium Bridge. It also makes regular donations to good causes through the City Bridge Trust. Were any of the five City bridges to collapse, the Bridge House Estates could draw on its reserves of around £500 million to rebuild.
We're reminded of the Bridge House Estates every time we cross the river and spot its illustrious mark. The Bridge Mark has been in use since at least the 17th century. The oldest surviving example can be found on a sword rest in Southwark Cathedral, which dates from 1674.
Its original meaning is lost to history, though some say it is emblematic of the martyrdom of St George — a version is associated with St George the Martyr church down Borough High Street.
Whatever its origins, the mark is now part of the fabric of London to such an extent that it even appeared in a recent episode of Peppa Pig.
An exhibition about the Bridge House Estates is on now at the Engine Rooms at Tower Bridge.