Counterbalance

Enchantment, UU (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.¹
Illus. John Zeleznik

Gatherer Card Rulings [Nov 2009]

• 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.

Format Legality	Legal in Vintage, Legacy, Extended, MTGO Classic Banned in Standard
Printings (1/9)	Coldsnap (Uncommon)