Well, natural progression, wouldn't that be set 1-4, TTT sites?
Is this actually a format though? Has anyone ever played sets 1-4 before?
I have, it was called Standard, when The Two Towers first came out.
However, as I said, it was STANDARD, we just didn't have other sets yet, so it WOULD be Towers Standard, sets 1-6, if we were to name the format NOW. In fact, the natural progression WOULD be Towers Standard FIRST, and THEN Towers Block. You play the first block, a new set comes out, you mix and match, learn to play with the new cards, see what old ones do not work that well anymore, get acquainted to the new set, and then it's time to kiss it goodbye, so you move to Towers Block. Thn, King's Standard, so on, so forth. However, to be fair, it doesn't HAVE to be this way, in fact, you either go Block by Block or Standard by Standard. THe natural progression would be King coming in and kicking FotR out of the picture, no block involved whatsoever, but it feels smoothier (not to mention, translates into more games) this way.
