Each player chooses their favorite block and builds a Block Constructed deck. The twist is that each player's favorite block may be different! That means the best decks from Mirrodin block will do battle against the likes of the decks from Onslaught block. Or you can see how the dual lands of Ravnica block would fare against the denizens of the Masques block. Players build decks according to the Block Constructed rules from each of their blocks (including their Banned Lists, if any) and play following the normal Block Constructed rules.