Magic: The Gathering Comprehensive Rules Changes - Core Set - Ninth Edition to Ravnica: City of Guilds

Old rule (Core Set - Ninth Edition)	New rule (Ravnica: City of Guilds)
X [Click to view all changes.]
X If a cost has an "X" in it, the value of X must be announced as part of playing the spell or ability. (See rule 409, "Playing Spells and Activated Abilities.") While the spell or ability is on the stack, the {X} in its mana cost equals the amount announced as part of playing the spell or ability. If a card in any other zone has {X} in its mana cost, the amount is treated as 0. If you're playing a spell that has X in its mana cost and an effect lets you play it without paying any cost that includes X, the only legal choice for X is 0. This does not apply to effects that only reduce a cost, even if they reduce it to zero. See rule 409, "Playing Spells and Activated Abilities." In triggered abilities, X is defined when the ability resolves. It may be defined by the text of the ability, by a keyword ability of the card, or by the trigger event. See rule 410, "Handling Triggered Abilities." In other cases, X is defined by the text of a spell or ability. If X isn't defined, the controller of the spell or ability chooses the value of X. All Xs on an object have the same value.	X If a cost has an "{X}" in it, the value of X must be announced as part of playing the spell or ability. (See rule 409, "Playing Spells and Activated Abilities.") While the spell or ability is on the stack, the {X} in its mana cost equals the amount announced as part of playing the spell or ability. If a card in any other zone has {X} in its mana cost, the amount is treated as 0. If you're playing a spell that has {X} in its mana cost and an effect lets you play it without paying any cost that includes X, the only legal choice for X is 0. This does not apply to effects that only reduce a cost, even if they reduce it to zero. See rule 409, "Playing Spells and Activated Abilities." In triggered abilities, X is defined when the ability resolves. It may be defined by the text of the ability, by a keyword ability of the card, or by the trigger event. See rule 410, "Handling Triggered Abilities." In other cases, X is defined by the text of a spell or ability. If X isn't defined, the controller of the spell or ability chooses the value of X. All Xs on an object have the same value.

Old rule (Core Set - Ninth Edition)

New rule (Ravnica: City of Guilds)

If a cost has an "X" in it, the value of X must be announced as part of playing the spell or ability. (See rule 409, "Playing Spells and Activated Abilities.") While the spell or ability is on the stack, the {X} in its mana cost equals the amount announced as part of playing the spell or ability. If a card in any other zone has {X} in its mana cost, the amount is treated as 0. If you're playing a spell that has X in its mana cost and an effect lets you play it without paying any cost that includes X, the only legal choice for X is 0. This does not apply to effects that only reduce a cost, even if they reduce it to zero. See rule 409, "Playing Spells and Activated Abilities." In triggered abilities, X is defined when the ability resolves. It may be defined by the text of the ability, by a keyword ability of the card, or by the trigger event. See rule 410, "Handling Triggered Abilities." In other cases, X is defined by the text of a spell or ability. If X isn't defined, the controller of the spell or ability chooses the value of X. All Xs on an object have the same value.

If a cost has an "{X}" in it, the value of X must be announced as part of playing the spell or ability. (See rule 409, "Playing Spells and Activated Abilities.") While the spell or ability is on the stack, the {X} in its mana cost equals the amount announced as part of playing the spell or ability. If a card in any other zone has {X} in its mana cost, the amount is treated as 0. If you're playing a spell that has {X} in its mana cost and an effect lets you play it without paying any cost that includes X, the only legal choice for X is 0. This does not apply to effects that only reduce a cost, even if they reduce it to zero. See rule 409, "Playing Spells and Activated Abilities." In triggered abilities, X is defined when the ability resolves. It may be defined by the text of the ability, by a keyword ability of the card, or by the trigger event. See rule 410, "Handling Triggered Abilities." In other cases, X is defined by the text of a spell or ability. If X isn't defined, the controller of the spell or ability chooses the value of X. All Xs on an object have the same value.