Flipping a coin is method of randomization of effects with two possible outcomes of equal likelihood.
The coin flip was introduced as a mechanic in Arabian Nights with Mijae Djinn, Ydwen Efreet and Bottle of Suleiman. Coin flips are mainly used on red cards and artifacts. If the card is multicolored, it's always red/blue. There is one black card with the mechanic (Tavern Swindler) and one green Un-card (Flock of Rabid Sheep).
From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 705. Flipping a Coin
- 705.1. To flip a coin for an object that cares whether a player wins or loses the flip, the affected player flips the coin and calls “heads” or “tails.” If the call matches the result, that 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. To flip a coin for an object that cares whether the coin comes up heads or tails, each affected 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.”