You may have had it hammered into you that “X is always 0 for converted mana cost!” While this is certainly true for objects on the battlefield and for cards in your hand, graveyard, library, and exile, it’s not true for spells on the stack. That’s the one place that “X” has a numerical value other than 0. As you cast an X spell, you choose a value for X, then pay mana based on that choice. Since you rarely cast an X spell choosing 0 for X, it’s pretty rare for X to be 0 for a spell on the stack.

For example, if you cast Clash of Wills with X=3, then the spell’s mana cost is 3U for a CMC of 4. It won’t trigger Eidolon of the Great Revel, and it can be countered by Disdainful Stroke.

A spell with two X’s in the cost is treated no differently. You choose a value for X, and each X is the same. To determine CMC for that spell, you add the two values together. So if you cast Hangarback Walker for X=1, you get a CMC of 1+1=2, and it would trigger Eidolon but dodges Disdainful Stroke, while X=2 means a CMC of 2+2=4, evading Eidolon’s trigger but becoming vulnerable to Disdainful Stroke.

Today’s Rules Tip written by Josh Stansfield