Fabric can be any good quality quilting cotton (our rule of thumb is "send what you'd like to receive"), but no flannels. No special theme.
We swap 6.5" squares. If you back the Cubs, and they lose to the Cardinals, you'll owe each Cardinal backer a square. If the Cubs win, they'll each owe you a square. The games aren't netted out: if the Cubs win 8 games and lose
4 to the Cards, you get 8 squares from and send 4 to each Cardinals backer.
We settle up twice a year -- after the All Star Break and after the season is over. When it's time to settle up, you get a lovely report telling you how many squares you owe to each person in the swap, complete with labels. You bag them up, send them to a "flinger" (me this year, but it rotates), who sorts them and sends your winnings back.
Lots and lots of fun -- I hope we'll see you there!