When travelling to London I use the London Underground, or, depending on my destination, the Overground. Because of this I have visited many stations, with their countless escalators, lifts, staircases, corridors and ticket halls, which make each station unique in its own way.
Another thing that they all share are small blue number plates, affixed to walls and doors throughout the station complex. Some corridors can have multiple SIDs, depending on their length.
Each plate as a single digit number above a three digit number. The top number is easy to work out as this denotes the level beneath ground, but the longer number is a lot trickier to understand.
