When you check in, you are given a boarding card with A, B or C on it. Group A boards first, then B, then C. There may be pre-boards before group A (ie, those in wheelchairs, needing special assistance, or children under a certain age - I think 5). Basically, if you have an A card, you get in the A line. First come, first served with seating. Personally, I much prefer this - I don't have to spend a flight sitting next to anyone I don't want to.
I *believe* the boarding card letter is based on when you check in. However, on Sunday starting at 12:01 am, you can check in online at
www.southwest.com and print your boarding card there.
I'll be flying Southwest (for the millionth time, lol) on Saturday and Sunday. I love 'em.
Kimya