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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 mana value as the revealed card.
If an opponent casts half of a split card (for example, Hit), it won’t be countered if you reveal that same split card. The converted mana cost of a split card is determined by the combined mana cost of its two halves, but the converted mana cost of Hit is determined using only that half’s mana cost.
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.
