What Are The Blue And White Numbered Signs At Tube Stations?

Laura Reynolds
By Laura Reynolds Last edited 21 months ago
What Are The Blue And White Numbered Signs At Tube Stations?
This one's at Old Street

Recognise these blue signs? If you've ever got the tube, anywhere, ever, you should do. They exist by the bucketload in tube stations, but what do their seemingly random numbers mean? Why are they there? What do they do? Does anyone know?

We touched on the issue briefly back in 2011 (clearly, this one's been puzzling us for a while), when we said:

"they’re code numbers for the emergency services, making it easier for rescuers to orient themselves in smokey conditions. The top number on each sign denotes the level below ground, while the remaining numbers define a location on that level."

So the signs are Station Identification Numbers (SID). Each station has a layout diagram, with the numbers allowing for very specific locations to be identified, e.g. for contractors to perform work and maintenance. At the entrance to each tube station there is also a red London Fire Brigade emergency box, which is opened in an emergency and contains station plans using these numbers.

In some instances you can also find these signs which have letters instead, instead of numbers. There is an "A/101" sign at Hounslow West, an anomaly to the usual convention of the first number relating to the level of the station - it's because when you go up from street level, each level uses letters instead of numbers, 'A' for the first level up, 'B' for the second level up, and so on.

Seen any other blue signs on the tube? Know any more about what they mean? Let us know in the comments.

Last Updated 16 August 2016


The 'A' of the A/101 sign will still refer to its level. The first part of an asset number is either a number or a letter depending on whether it is above ground/entrance level, or whether it is below.


They are known as SIDS or Station Area Identification Signs. Marckee is right, numbers for below ground letters for above. Hounslow West is definitely not the only station to have an 'A' SIDS some even have ones with B on it!


X/6XX is generally for stairs.

Peter Seddon

When I first started working on London Underground, I was told that, as previously mentioned, the first character was for the level, lower number was higher, and higher number was lower when under ground, lower alpha was lower and higher alpha was higher above ground. The other 3 digit number was always specific to the room behind the door, ie if the number was 123 and was a ticket office for example the every 123 would be a ticket office. However, the number was very precise, 123 maybe a ticket office with a mess-room and 124 might be a ticket office with no mess room, but you could expect to find the same thing behind each door with the same 3 digit number. Hence on any single stations plans the numbers would be unique to the station and would the emergency services know what to expect>


Shhhhhh! You can't share this information publicly. We'll need to rely on these codes for when the aliens attack and we go underground for shelter. Delete it!