A "Chow" is three consecutive tiles of the same suit.
You are ONLY allowed to collect Chows from the suits, Circles, Bamboo or Wan.
There are three ways to make a Chow:
By picking up a discarded tile BUT ONLY if the tile has been discarded by the player to the immediate left of you, if you make a Chow this way it is called an Exposed Chow and is displayed above your stake for all other players to see.
You can make Chows if you wish from tiles you have been dealt. If you do this the Chow can stay in your hand, in such a case it is known as a Concealed Chow.
You can make a Chow to go Mah Jongg, in such cases you are allowed to pick up a tile that has been discarded by any player in order to make the Chow and go Mah Jongg.
There are some circumstances when you are not allowed to pick up a discarded tile to make a Chow - this occurs when two or more players want to pick up the same discarded tile. See Precedences for more information.
See also Rules.