Sometimes X appears in the text of a spell or ability but not in a mana cost, alternative cost, additional cost, or activation cost. If the value of X isn’t defined, the controller of the spell or ability chooses the value of X at the appropriate time (either as it’s put on the stack or as it resolves).
September 30, 2011 · Innistrad
If a card in any zone other than the stack has an {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.
June 17, 2011 · Commander 2011
If a card in any zone other than the stack has an {X} in its mana cost, the value of {X} is
February 4, 2011 · Mirrodin Besieged
If a card in any zone other than the stack has an {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.
October 1, 2010 · Scars of Mirrodin
If a card in any zone other than the stack has an {X} in its mana cost, the value of {X} is
February 1, 2010 · Worldwake
If a card in any zone other than the stack has an {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.