Clones are cards that copy another permanent, most commonly a creature, allowing you to duplicate the most useful thing already on the battlefield. Clone decks can turn an opponent's best threats against them or repeatedly copy their own value engines.
Some of the most notable Clone cards include the original Alpha Clone copying a creature, Phyrexian Metamorph copying either an artifact or creature, and Spark Double copying one of your creatures or planeswalkers without retaining the legendary supertype, allowing you to “Double” up.
