X

X is a variable; a number that needs to be determined. On a Magic card, it can appear can 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

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

X

A placeholder for a number that needs to be determined. See rule 107.3.

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 107.3. Many objects use the letter X as a placeholder for a number that needs to be determined. Some objects have abilities that define the value of X; the rest let their controller choose the value of X.
  • 107.3a 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.
  • 107.3b If a player is casting a spell that has an {X} in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn’t apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, “Casting Spells.”
  • 107.3c If a spell or activated ability has an {X}, [-X], or X in its cost and/or its text, and the value of X is defined by the text of that spell or ability, then that’s the value of X while that spell or ability is on the stack. The controller of that spell or ability doesn’t get to choose the value. Note that the value of X may change while that spell or ability is on the stack.
  • 107.3d If a cost associated with a special action, such as a suspend cost or a morph cost, has an {X} or an X in it, the value of X is chosen by the player taking the special action immediately before they pay that cost.
  • 107.3e 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.
  • 107.3f 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).
  • 107.3g 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.
  • 107.3h If an effect instructs a player to pay an object’s mana cost that includes {X}, the value of X is treated as 0 unless the object is a spell on the stack. In that case, the value of X is the value chosen or determined for it as the spell was cast.
  • 107.3i Normally, all instances of X on an object have the same value at any given time.
  • 107.3j If an object gains an ability, the value of X within that ability is the value defined by that ability, or 0 if that ability doesn’t define a value of X. This is an exception to rule 107.3i. This may occur with ability-adding effects, text-changing effects, or copy effects.
  • 107.3k If an object’s activated ability has an {X}, [-X], or X in its activation cost, the value of X for that ability is independent of any other values of X chosen for that object or for other instances of abilities of that object. This is an exception to rule 107.3i.
  • 107.3m If an object’s enters-the-battlefield triggered ability or replacement effect refers to X, and the spell that became that object as it resolved had a value of X chosen for any of its costs, the value of X for that ability is the same as the value of X for that spell, although the value of X for that permanent is 0. This is an exception to rule 107.3i.
  • 107.3n If a delayed triggered ability created by a resolving spell or ability refers to X, X is not defined in the text of that triggered ability, and the spell or ability that created it had a value of X chosen for any of its costs, the value of X for the triggered ability is the same as the value of X for the spell of ability that created it.
  • 107.3p Some objects use the letter Y in addition to the letter X. Y follows the same rules as X.

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 Fireball, was only used on the silver-bordered The Ultimate Nightmare of Wizards of the Coast® Customer Service.
• 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 Cornucopia is the only card with a cost of .