Given that my standard public transport route to work (Walk-Train-Bus/Walk) avoids the tube, and that I recently started cycling in on most days, I don't travel on the tube much. However, I agree with Meg Pickard that there is a definite hierarchy of seats within each carriage.
In the reference diagram below, the preferred pecking order of one end of a Hammersmith & City line carriage is dissected:

Position 1 is supreme, because you have only one
neighbour, and (usually) a wider seat, with windows behind and beside
you, plus a door which might give some hope of a breeze.
Position 2 ...
I disagree about position 10. When I take the tube, I go for 10 immediately if there are no seats as you have a much better chance of getting a seat that you do at 6, 7 or 8. People who sit at 3 are not going far so they usually get out a stop or two after you get on. if you are in 9 you won't get the seat at 3 and potentially not at 4 either. 10 should be at least 6
Posted by: Renee | Friday, 03 April 2009 at 23:15