Reading the City

You know the way you see things every day but don't fully understand what they signify? Everyone has seen the 'H' signs all over the UK, and I did know (or at least was fairly sure) they were something to do with water. 'H' for 'Hydrant'. For the fire brigade to link into when they need access to water. But I was never clear what the numbers were about. So today I looked them up and apparently the top number is the diameter of the water main - in millimetres these days, although previously in inches - and the bottom number is the distance to the hydrant from the sign. So there is a 300mm main 2 metres from this sign. Apparently other countries have similar signs. for example they have blue ones in Finland, and the Australian ones also indicate whether the water is drinkable or not.
You learn something every day.