X

X is a variable; a number that needs to be determined. On a Magic card, it can appear in three different locations:
 * A variable in the mana cost of a spell or ability, i.e..
 * A variable in the activation cost of an ability or the additional costs of a spell, such as removing X loyalty counters or sacrificing X creatures.
 * A variable in the effect of a spell or ability, such as "gain X life" or "deal X damage". This is usually determined by the X value given to one or both of the aforementioned variables.

Rules
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Rulings

 * If a spell or ability has a cost with an "" 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, the only legal choice for  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 "" in it, the value of 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.
 * A mana cost of means that you pay twice X. If you want X to be 3, you pay 6 for the  in the spell's mana cost. Likewise, a mana cost of  means that you pay three times X, and so forth.

Trivia

 * Phyrexian Marauder is the first card to ever have a mana cost of just.
 * The mana cost symbols and  (with the same function as ) are obsolete.    has been used on the Beatdown Box printing of,  was only used on the silver-bordered The Ultimate Nightmare of Wizards of the Coast® Customer Service and the test card Experiment Five.
 * X is the expansion symbol for Tenth Edition.
 * The character X is the master spy of the Agents of S.N.E.A.K..
 * Astral Cornucopiaand are the only cards with the cost of.
 * Unbound Flourishing doubles the value of X.
 * Wizards of the Coast R&D is part of Studio X.