If a spell or ability refers to the {X} or X in the mana cost, alternative cost, additional cost, or activation cost of another object, any X in that spell or ability’s text uses the value of X used by the other object.
September 25, 2020 · Zendikar Rising
If a spell or ability refers to the {X} or X in the mana cost, alternative cost, additional cost, or activation cost of another object, any X in that spell or ability’s text uses the value of X chosen or defined for the other object.
May 3, 2019 · War of the Spark
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).
January 25, 2019 · Ravnica Allegiance
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).
February 1, 2014 · Born of the Gods
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
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).
June 17, 2011 · Commander 2011
Sometimes X appears in the text of a spell or ability but not in a mana cost, alternative cost,
April 1, 2011 · Duel Decks: Knights vs. Dragons
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).
February 4, 2011 · Mirrodin Besieged
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).
October 1, 2010 · Scars of Mirrodin
Sometimes X appears in the text of a spell or ability but not in a mana cost, alternative cost,
February 1, 2010 · Worldwake
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).