MTG FORUM

So, the way the X rule works is that you typically declare a value for X lets say 5, and then pay 5 mana, along with the cost of the spell. For example, if a card says 'Deals X damage' and costs {X}{R} you'd pay 5 and one red to deal 5 damage.

Also, for the purposes of deciding a cards Mana Value, X is treated as 0 although, when deciding a spells Mana Cost, X is whatever you paid, in most cases X cannot be zero and if an X spell is copied, X retains its value within the copy.

Furthermore, there are also a handful of cards that have {X}{X} or even {X}{X}{X} in their mana costs in theses cases, if you wanted {X} to be five, you'd have to pay 10 and 15 respectively.