Double

Double is a keyword action formally introduced in Dominaria for Grunn, the Lonely King.

Description
Double multiplies certain characteristics or conditions by two. Doubling has existed informally as early as Fifth Dawn with cards like Doubling Cube and Solarion.

Doubling may be applied to:
 * The number of counters (e.g. Deepglow Skate, Kalonian Hydra)
 * The amount of each type of mana in your mana pool (Doubling Cube)
 * Power and toughness (Grunn, the Lonely King)
 * Life total (Beacon of Immortality)
 * X Values (Unbound Flourishing</c>)

Rules
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Trivia

 * Nyxbloom Ancient</c> and Fiery Emancipation</c> triple effects.
 * Mark Rosewater loves to design doubling mechanics.
 * Maro's Gone Nuts</c> is a Mystery Booster test card that spoofs this. It doubles any effect that doubles (it quadruples).