Mahjong has been adopted and tweaked by various cultures and countries over the centuries. Today numerous styles of Mahjong are played across the world. The basic rules remain fairly constant with variations in tile designs, number of tiles played and scoring. With basic Mahjong knowledge it is convenient to hop from one style to another.
For more information on Mahjong styles, please refer to http://Mahjong.wikidot.com/list-of-rules