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.
許多物件會使用字母X來代表一個尚待定值的數字。有些物件的異能會去定義X的值；其它的就由其操控者來選擇X的值。
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 equals the announced value. While an activated ability is on the stack, any X in its activation cost equals the announced value.
如果一個咒語或起動式異能，其魔法力費用、替代性施放費用、額外費用、和／或起動費用包括了｛Ｘ｝，[-X]、或是X，且X的值並未在咒語或異能中的文字定義的話，便由該咒語或異能的操控者選擇並宣告X的值，以作為施放咒語或起動異能的一部分。（見規則601，「施放咒語」。）咒語在堆疊上時，魔法力費用中的X會等同於先前被宣告的值。起動式異能在堆疊上時，起動費用中的X會等同於先前被宣告的值。
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.”
若玩家施放了一個魔法力費用帶｛Ｘ｝的咒語，且X的值沒被咒語的文字定義，而有效應讓該玩家施放咒語時不需支付魔法力費用，或是有沒定義X的替代性施放費用的話，X的唯一合法選擇就是0。此規則不適用於減少費用的效應，哪怕該效應把費用減到零也一樣。見規則601，「施放咒語」。
這時常常被問到的問題「免費施放 vs. X咒語」。答案很簡單，X就是0。
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.
若咒語或起動式異能，其費用和／或其文字欄包括了｛Ｘ｝，[-X]、或是X，且X的值在咒語或異能的文字中被定義了，那麼X的值即為該咒語或異能在堆疊上的值。咒語與異能的操控者無法選擇其值。注意咒語或異能在堆疊的過程中，X的值可能變化。
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 as he or she pays that cost.
若費用與特殊行動相關，比如延緩費用或是變身費用，若包括了｛Ｘ｝或是有個X在其中，X的值便由採取特殊行動的玩家，在支付費用時決定。
107.3e 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).
有時X會出現在咒語或異能的文字欄中，但不代表魔法力費用、替代性費用、額外費用、或是起動費用。如果X的值未定義，該咒語或異能的操控者要在適當的時機去選擇X的值（不是進堆疊時就是結算時）。
通常是結算時，進堆疊的就要選的話那應該是跟「目標」相關。
107.3f 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.
一張牌在堆疊以外的其它地方，其魔法力費用中的｛Ｘ｝可以視為0，哪怕X的值在其文字有被定義。
以原初多頭龍為例，能不能倨傲擊視X而定，但不管X多少上了戰場一定可以悶殺。
107.3g All instances of X on an object have the same value at any given time.
在任意時間點下，一個物件裡所有的X，其值均相等。
107.3h Some objects use the letter Y in addition to the letter X. Y follows the same rules as X.
有些物件除了字只X外還會使用字母Y。Y的定義規則與X相同。
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