I was shocked the other day by seeing a physical travelcard ticket for Zones 1-9 and having to confirm from a Tube station map that a Zone 9 actually exists. This is four more than I could remember.
This post really highlights how much of the development in London is infill into already existing Victorian (or earlier) neighbourhoods and how the city is growing without sprawling outwards. The Victorians built the Metropolitan line out almost all the way to Aylesbury anticipating that the city would follow but it really hasn't. When you take the tube to zone 9 you start to go past horses and cows in fields. There are some really nice walks in the Chiltern hills starting from Chesham and Amersham and you don't feel like you're in London at all. We always wonder if people living there even consider themselves as Londoners.
Zones beyond 6 have existed since at least the late 90s. My vague recollection is that A-C basically meant "the top end of the Metropolitan line" - certainly how it seemed for me as an SW Londoner. Then at some point the letters were changed to numbers and more stations were brought inside over in Essex, Kent, etc. The Wikipedia page for stations in zones 7-9 has a changelog of sorts: https://en.wikipedia.org/wiki/List_of_stations_in_London_far...
Long ago when I moved to London, it was zones 1-6 then A-D for stations outside Greater London. I think changing A-D to 7, 8 & 9 made it simpler; most people aren't interested in what the local government is when they buy a ticket.
To be fair I didn't know that either... For some reason I thought 7 was the last zone... 13 years here (almost to the day) and I've learned something new !
