Q: Why are house numbers odd on one side of the road, and even on the other?
A: Well, they aren’t always. But where they are, odd numbers tend to be on the left as you face away from the nearest town centre. And the numbers go up the further from the town centre you get.
And how is a town centre defined? Well, in this instance, it’s the location of the post office. So effectively this system makes things easier for the posties when they’re out on their rounds.
But wait... What about streets which go in sequence?
There are, of course, streets where the numbers go up sequentially on one side, and come back down on the other side. Downing Street is one example. And house numbers tend to go up clockwise around squares.
The problem for sequential streets is: what happens if planners wish to extend them? When house numbers are odd on one side and even on the other, the solution is simple.
Fun Fact: Some streets even have a missing number 13, thanks to triskaidekaphobic* local authorities.