Source: Yugioh Wikia

A 'Chain is formed when one player activates a Card effect and his/her opponent responds with an effect of his/her own. For example:

A Chain is resolved starting with the last card played and working backward. For example:

Player 1's "Seven Tools of the Bandit" negates Player 2's "Magic Jammer".

"Magic Jammer's" effect is negated, however, Player 2 still has to pay the 1 card required as a discard cost from his hand, for the activation of "Magic Jammer", even though its effect was negated. The chain ends with all three cards going to the Graveyard, plus 1 card from Player 2's hand, and "Raigeki's" effect taking place.

A Chain is started whenever someone activates a card. That card automatically becomes Chain Link 1. If it goes unopposed, the chain still resolves with only one step having been taken.

Declaring an Attack or Summoning a Monster cannot be chained to, since those actions do not possess a Spell Speed. Instead, those actions can only be responded to if it was the last action to have happened on the field. If the Summon or Attack isn't responded to by the opponent (by passing, for example), then a player cannot then use a card that can only be activated in response to a Summon or Attack, such as "Trap Hole" or "Bottomless Trap Hole" (for a Summon) or "Mirror Force" or "Sakuretsu Armor" (for an attack), since the timing has been missed.

Example 1: Player 1 Summons a "Blue-Eyes White Dragon, then in response Player 1 activates "Rush Recklessly" on him. A chain has been started, but therefore the Summon is still the last action to have resolved on the field, and you can still chain "Trap Hole" to "Rush Recklessly

Example 2: Player 1 summons a "Blue-Eyes White Dragon", but Player 2 declines to activate a card in response. If Player 1 then enters the Battle Phase and declares an attack, Player 2 has obviously missed the chance to activate in response to the summon and "Trap Hole" cannot be activated.

You can chain to every monster effect with the exception of Continuous Effects, that come into play as soon as the monster hits the field (Examples: "Jinzo]" and "Spell Canceller").

In a Chain, the effects are never activated immediately, until the last link in the chain has been declared. Then, starting from that last link, and moving back, the effects are activated in reverse order. Take this example:

Here, "Dust Tornado" would activate first, without "Royal Decree's" trap-negating effect going off. The same situation and result would occur if:

Also, you can chain to your own cards. Here is one such example:

This is actually a legal move; "Magic Cylinder" would negate the attack, and inflict burn damage to Player 1. Then, "Mirror Force" would resolve, and since its effect requirements have still been fulfilled (namely, a monster has still declared an attack, and there is at least 1 face-up Attack Position monster on the other side of the field), it would destroy all Player 1's Attack Position monsters.

It is entirely possible to win a game Mid-Chain.

Here, "Ring of Destruction" would resolve first, destroying "Mobius the Frost Monarch" to inflict 2400 points of damage to both players. Player 1 would have 100 Life Points remaining, but since Player 2's Life Points would drop to 0, he would lose the match, before his "Magic Cylinder" would take effect.

However, you cannot declare a win in the middle of a Chain Link. Here is one such example:

Here, Player 1 must discard two cards for the effect of "Graceful Charity", and cannot declare a Victory until the Chain Link has fully resolved.

One should be careful though about the difference between simply destroying a card and negating the effects of that card. For example, activating "Mystical Space Typhoon" on "Magic Cylinder" would not stop the damage being reflected back as "Mystical Space Typhoon" does not say 'Negate the effects of...' therefore you have more or less wasted the use of a card.

However, if a card must remain on the field for its effects to work, such as "Call of the Haunted", the effects of that card are negated as the card no longer exists on the field.

See Also: Chains, Activation, and Resolution.
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