Rules glossary
↑ Parent rule: 107.3 Many objects use the letter X as a placeholder for a number…
107.3h If an effect instructs a player to pay an object’s mana cost that includes {X}, the value of X is treated as 0 unless the object is a spell on the stack. In that case, the value of X is the value chosen or determined for it as the spell was cast.
No sub-rules.