Counterbalance

Enchantment    (CMC:2)

Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, counter that spell if it has the same converted mana cost as the revealed card.

31 CSP • ENJohn Zeleznik

™ & © 1993—2006 Wizards of the Coast

Legal in: Modern,Ice Age Block,Legacy,Vintage,Commander

Oracle Text (click to copy):

Counterbalance {U}{U} Enchantment Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, counter that spell if it has the same converted mana cost as the revealed card.

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View this MTG card on Gatherer

7/15/2006
If an opponent casts a spell with X in its mana cost, the converted mana cost of that spell takes the value of X into account. If you reveal a card with X in its mana cost, X is 0. For example, if your opponent casts Blaze with X=1, Counterbalance will counter that spell if you reveal a card with converted mana cost 2, but it won't counter that spell if you reveal a Blaze of your own.
7/15/2006
If an opponent casts half of a split card (for example, Hit), and you reveal a split card where half of it has the same converted mana cost (for example, Hit/Run or Stand/Deliver), Counterbalance will counter the spell.

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Card Counterbalance is not on TCGPlayer.