A major part of American mahjong is calling for discarded tiles. 
Let’s talk through:
- How do you call a tile in mahjong?
- What happens if you say pause versus call?
- What if two players both want the same discarded tile at the same time?
In mahjong, the most recently discarded tile is called the live tile. That tile is the one that is available for players at the table to call and make part of their hand. If you want to call a tile, you must be able to show the full grouping of those tiles as part of your hand on top of your rack. It must be a grouping of three or more identical tiles in order to be exposed. Once you expose a grouping on your rack, you are then locked in with that grouping for the game; this means that if you are going to win, you must have that exposure as part of your hand to win the game.
Players can say pause or hold to give themselves a bit more time to think about whether or not they can call/take a tile. Saying pause does not commit a player to taking the tile. It just gives that player more time to think before the next player draws and racks their tile. If the player who paused, decides they do want the discarded tile they’ll say call and then expose their tiles to show the full grouping.
If two players want the same tile at the same time, there is a special pattern of rules to determine who gets it, almost like a waterfall. If one player can win mahjong with the called tile, then that player gets it. If neither or both players can win mahjong with the called tile, then the player next in turn gets the tile. Unless one player has already started to expose the tile that go with that grouping, then the player with the exposed tiles gets the called tile.
Happy mahj-ing y’all! 
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- Umbrella Playing Mat: https://southernsparrow.com/products/mahjong-game-mat-umbrella-in-pink-yellow