Counterbalance
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.
Illustrated by Joseph Meehan
This printing
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If an opponent casts a spell with X in its mana cost, the mana value 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 mana value 2, but it won't counter that spell if you reveal a Blaze of your own.
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 mana value of a split card is determined by the combined mana cost of its two halves, but the mana value of Hit is determined using only that half's mana cost.
