Mark Rosewater has described randomness as a state that, if not interfered with, has unpredictability. Richard Garfield states that any game whose outcome is not a foregone conclusion has a degree of luck.
Randomness is first introduced to the game when each player shuffles their libraries before the game. While the Comprehensive Rules state only that libraries must be in a state such that no player knows their order, the Tournament Rules go into substantially more detail. Notably, the Tournament Rules state that pile shuffling alone is not adequate. Additionally, players must have the option of performing a final shuffle of their opponent's deck. At the Competitive and Professional Rules Enforcement Levels, players must shuffle their opponents decks.
When flipping a coin, other sources of randomness are allowed, as long as there are two outcomes of equal likelihood and all players agree to that substitution.
- 701.20a To shuffle a library or a face-down pile of cards, randomize the cards within it so that no player knows their order.
- 705. Flipping a Coin
- 705.1. An effect that instructs a player to flip a coin may care whether that player wins or loses the flip. To flip a coin for such an effect, the player flips the coin and calls “heads” or “tails.” If the call matches the result, the player wins the flip. Otherwise, the player loses the flip. Only the player who flips the coin wins or loses the flip; no other players are involved.
- 705.2. If an effect instructs a player to flip a coin and that effect cares only whether the coin comes up heads or tails without specifying a winner or loser of the flip, that player flips a coin without making a call. No player wins or loses this kind of flip.
- 705.3. A coin used in a flip must be a two-sided object with easily distinguished sides and equal likelihood that either side lands face up. If the coin that’s being flipped doesn’t have an obvious “heads” or “tails,” designate one side to be “heads,” and the other side to be “tails.” Other methods of randomization may be substituted for flipping a coin as long as there are two possible outcomes of equal likelihood and all players agree to the substitution. For example, the player may roll an even-sided die and call “odds” or “evens,” or roll an even-sided die and designate that “odds” means “heads” and “evens” means “tails.”