American Mah Jongg is considered the most difficult style because only the hands listed on a card are considered valid. This requires players to memorize the card (or at least have a good grasp of it).
o The "1-4-7 rule" is a good playing strategy (for all forms of Mah-Jongg except American (style similar to NMJL) in which there are no "chows"). If the player to your right discards a 4, and you don't have another of those to discard, you /might/ be all right if you discard a 1 or a 7.