General Rulings - G24

• G24.1a - Many cards use the letter X as a placeholder for a number that needs to be determined. All instances of X on an object have the same value. [CompRules 2008/07/15]
• G24.1b - If a spell or activated ability has a cost with an "{X}" in it, and the value of X isn't defined by the text of that spell or ability, the controller of that spell or ability chooses and announces the value of X as part of playing the spell or ability. (See Rule 409, "Playing Spells and Activated Abilities.") While the spell or ability is on the stack, the {X} in its mana cost equals the announced value. [CompRules 2008/07/15]
• G24.1c - If you're playing a spell that has {X} in its mana cost, the value of X isn't defined by the text of that spell, and and effect lets you play that spell without paying any cost that includes {X}, then the only legal choice for X is 0. This doesn't apply to effects that only reduce a cost, even if they reduce it to zero. See Rule 409, "Playing Spells and Activated Abilities." [CompRules 2008/07/15]
• G24.1d - If a cost associated with a special action, such as a suspend cost or a morph cost, has an "{X}" in it, the value of X is chosen by the player taking the special action as he or she pays that cost. [CompRules 2007/05/01]
• G24.1e - If a card in any zone other than the stack has {X} in its mana cost, the value of {X} is treated as 0, even if the value of X is defined somewhere within its text. [CompRules 2008/07/15]
• G24.1f - In other cases, X appears in the text of a spell or ability but not in a mana cost or activation cost. If the value of X is defined by the text of that spell or ability, then that's the value of X while that spell or ability is on the stack. The controller of that spell or ability doesn't get to choose the value. Note that the value of X may change while that spell or ability is on the stack. If the value of X isn't defined, the controller of the spell or ability chooses the value of X. [CompRules 2008/07/15]
• Note - Also see Rule 409.1b, "Playing Spells and Activated Abilities."