Rule evolution
Rule 107.3a
Earliest archived Rules release: 2009-10-01 · 1 wording revision · 0 renumberings
Earliest archived Rules release107.3aDetails
If a spell or activated ability has a mana cost, alternative cost, additional cost, and/or activation cost with an {X}, [-X], or 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 casting the spell or activating the ability. (See rule 601, "Casting Spells.") While a spell is on the stack, any X in its mana cost equals the announced value. While an activated ability is on the stack, any X in its activation cost equals the announced value.
Wording changed107.3aDetails
If a spell or activated ability has a mana cost, alternative cost, additional cost, and/or activation cost with an {X}, [-X], or 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 casting the spell or activating the ability. (See rule 601, "Casting Spells.") While a spell is on the stack, any X in its mana cost or in any alternative cost or additional cost it has equals the announced value. While an activated ability is on the stack, any X in its activation cost equals the announced value.