If a spell or ability has a cost with an “{X}” in it, the value of X must be announced 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 in its mana cost equals the amount announced as part of playing the spell or ability. If a card in any other zone has in its mana cost, the amount is treated as 0. If you’re playing a spell that has in its mana cost and an effect lets you play it without paying any cost that includes X, the only legal choice for X is 0. This does not apply to effects that only reduce a cost, even if they reduce it to zero. See rule 409, “Playing Spells and Activated Abilities.” 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. In triggered abilities, X is defined when the ability resolves. It may be defined by the text of the ability, by a keyword ability of the card, or by the trigger event. See rule 410, “Handling Triggered Abilities.” In other cases, X is defined by the text of a spell or ability. If X isn’t defined, the controller of the spell or ability chooses the value of X. All Xs on an object have the same value.