It is important to know that in mahjong the winds do not follow the cardinal directions (north, east, south, west) like on a compass. They follow the Asian cardinal points (east, south, west, north).
Seat winds are positions assigned during the seating phase of the game.
When the deal passes the seat winds shift counter-clockwise, making the new dealer east, the player left of the dealer south, the player across west, etc. After each player has had a chance to be the dealer once the round ends and a new round is started. A complete mahjong game consists of four rounds.
The prevalent wind (or round wind) is the wind designated for the round. For the first round the prevalent wind is east, for the second south, third west, and fourth north.
In general a player earns more points, a fan, or a double (depending on the scoring system of course) if he has a pung of his seat wind and/or prevalent wind. A player whose seat wind is also the prevalent wind has a double wind.
Here's a table of the first two rounds to try and illustrate the winds and prevalent winds if you are utterly confused.
|Prevalent Wind||- - - - - - - - - Seat Winds - - - - - - - - -|