The Middle Seat

I flew home from Atlanta yesterday, had a great flight, but I had to sit in the middle seat. I've never quite understood how airlines assign seating or how you end up with the boarding section. Air Tran boards by section and it doesn't seem to matter when you print your boarding pass, I was the 11th person to check in and still ended up in zone 6, and in row 15, which I quickly switched. Ended up in row 10 which is great, just behind business class with plenty of leg room, I just had to sit in the middle seat. There was a large man on the aisle and I was afraid that he was going to crowd me, but he didn't, a lady at the window, plenty of room on both sides...Great Flight!