Fat Stack is a casual limited Magic: The Gathering format where player use two pre-prepared stacks of cards that are used as communal libraries. The first stack features cards that can't produce mana, and the second stack features only lands and other cards that can produce mana (such as Dark Ritual, Birds of Paradise or Fellwar Stone).
Whenever a player draws a card, they may do so from either stack (all players know which stack is which). This includes the seven cards drawn for each player's opening hand. Choosing which stack of cards you'll draw from each turn is a fascinating challenge: Do you want to draw ways to produce mana that will help you play the cards in your hand, or do you want to draw cards that allow you to use the mana you can already produce? Players share both stacks of cards as a library and also use a communal graveyard. You can play Fat Stack as a two-player game, or as a multiplayer Free-for-All.
Many players enjoy Fat Stack because the game setup is very quick once someone has put together the stacks of cards to be used in a game, and it allows players to participate despite not having a large collection of Magic cards.