Rule 208.2a

The card may have a characteristic-defining ability that sets its power and/or toughness according to some stated condition. (See rule 604.3.) Such an ability is worded “[This creature’s] [power or toughness] is equal to . . .” or “[This creature’s] power and toughness are each equal to . . .” This ability functions everywhere, even outside the game. If the ability needs to use a number that can’t be determined, including inside a calculation, use 0 instead of that number. Example: Lost Order of Jarkeld has power and toughness each equal to 1+*. It has the abilities “As Lost Order of Jarkeld enters the battlefield, choose an opponent” and “Lost Order of Jarkeld’s power and toughness are each equal to 1 plus the number of creatures the chosen player controls.” While Lost Order of Jarkeld isn’t on the battlefield, there won’t be a chosen player. Its power and toughness will each be equal to 1 plus 0, so it’s 1/1.

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