Double is a keyword action formally introduced in Dominaria for Grunn, the Lonely King. It has existed informally as early as Fifth Dawn with cards like Doubling Cube and Solarion.
Doubling may be applied to:
From the Comprehensive Rules (Ravnica Allegiance (January 25, 2019))
- 701.9. Double
- 701.9a Doubling a creature's power and/or toughness creates a continuous effect. This effect modifies that creature's power and/or toughness but doesn't set those characteristics to a specific value. See rule 613.3c.
- 701.9b To double a creature's power, that creature gets +X/+0, where X is that creature's power as the spell or ability that doubles its power resolves. Similarly, an effect that doubles a creature's toughness gives it +0/+X, where X is that creature's toughness. Doubling a creature's power and toughness gives it +X/+Y, where X is its power and Y is its toughness.
- 701.9c If a creature's power is less than 0 when it's doubled, doubling that creature's power instead means that the creature gets -X/-0, where X is the difference between 0 and its power. Similarly, if its toughness is less than 0 when doubled, it gets -0/-X. If one characteristic's value is negative but the other isn't when both are doubled, it gets -X/+Y or +X/-Y, as appropriate.
- 701.9d To double a player's life total, the player gains or loses an amount of life such that their new life total is twice its current value.
- 701.9e To double the number of a kind of counters on a player or permanent, give that player or permanent as many of those counters as that player or permanent already has.
- 701.9f To double the amount of a type of mana in a player's mana pool, that player adds an amount of mana of that type equal to the amount they already have.