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A Pendulum Monster (Japanese: ペンデュラムモンスター Pendyuramumonsutā, abbreviated
While not in a Pendulum Zone, a Pendulum Monster is treated as a Monster Card, and its Monster Effects are applied, but its Pendulum Effects are not. If it is also a Normal or Effect Monster, it can be Normal Summoned/Set from the hand or Special Summoned by appropriate card effects, just like any other Normal or Effect Monster. If it is also a Synchro or Xyz Monster, it can be Synchro or Xyz Summoned (respectively) from face-down in the Extra Deck, just like any other Synchro or Xyz Monster.
While in a Pendulum Zone, it is treated as a Spell Card and not as a monster, as long as it remains in a Pendulum Zone. A Pendulum Monster can be activated from the hand as a Spell Card in your Pendulum Zone, but it is not possible for any cards to be Set in the Pendulum Zone. Upon resolving, it remains face-up in the Pendulum Zone as a Spell Card, and gains its Pendulum Effects, if any (its Monster Effects are not applied while treated as a Spell Card). Although a Pendulum Monster is treated as a Spell Card while in the Pendulum Zone, it is not considered a Normal, Continuous, Field, Equip, Quick-Play, or Ritual Spell Card. If a Pendulum Zone is already occupied, you cannot place a Pendulum Monster in it. As long as there are available Pendulum Zones, there is no limit to the number of times a Pendulum Monster can be activated from the hand per turn.
Once per turn, during the Main Phase, while there are Pendulum Monsters in both Pendulum Zones, the turn player can perform a Pendulum Summon, using the Pendulum Monsters' Pendulum Scales to determine the Levels of the monsters that can be Summoned.
When a Pendulum Monster (even if it is face-down) would be sent from the field to the Graveyard, either as a Monster Card or Spell Card, it is placed face-up in the Extra Deck Zone instead. (Pendulum Monsters can cause the total size of the Extra Deck to be larger than would be allowed during Deck building.) A Pendulum Monster is sent to the Graveyard like normal if it is sent from any location other than the field (specifically, if it would be sent from the hand, Main Deck, or Extra Deck to the Graveyard; has its activation as a Spell Card or Summon negated; is detached from an Xyz Monster; or the Xyz Monster it is attached to leaves the field). If the effect of a card like "Macro Cosmos" (which banishes cards which would be sent to the Graveyard) or "Grave Protector" (which shuffles monsters destroyed by battle into the Deck) is applying, this will override the Pendulum Monster's ability to go to the Extra Deck. A face-up Pendulum Monster in the Extra Deck Zone can be Summoned again by Pendulum Summon, as long as its Level is within the Scales.
As the Pendulum Effects and the Pendulum Scales take up the space where the Set Number would usually be placed, it is placed in the bottom left corner of the card instead, in the same line as the ATK/DEF values of the Pendulum Monster. The art frame of a Pendulum Monster is increased in size, to the same width as the Effect boxes, and the Pendulum Effect box is partially transparent, allowing the art to be seen beneath it.
A good Pendulum-based Deck has the ability to do what the anime calls "going beyond the Pendulum", which involves using Pendulum Summoning to gather Materials for a Fusion/Synchro/Xyz Summon, as Tributes for a Tribute Summon/Ritual Summon, or to activate multiple effects that activate when Summoned.
Pendulum Monsters' main weakness is their hand advantage cost to place in the Pendulum Zone, causing them to be vulnerable to effects such as those of "Black Rose Dragon" and "Evilswarm Exciton Knight". These "nuke" effects will destroy Pendulum Monsters in the Pendulum Zones as well as any Pendulum Monsters on the field, which will shut down any deck that relies on Pendulum Summoning until new Pendulum Scales can be placed. Furthermore, cards like "Hand Destruction" and "Solemn Warning" that can cause the Pendulum Monsters to be sent to the Graveyard can be problematic as well, since most Pendulum Monster Decks don't have ways to get cards back from the Graveyard, though you can remedy to this with cards like "Pot of Riches".
Cards that banish other cards (like "Divine Knight Ishzark" and "Chaos Sorcerer"), cards that specifically counter Spell Cards (like "Spell Canceller"), and cards that deplete the Extra Deck (like "Zaborg the Mega Monarch" and "Extra Gate) can also be problems for Pendulum-focused Decks.
Pendulum-based Decks also require that you either open with a sufficient amount of monsters or with the ability to search for them. If you don't open with enough monsters, Pendulum Summoning doesn't provide much of an advantage.
In the anime, Yuya's Pendulum Monsters were originally Effect Monsters, but they were transformed by the power of Yuya's pendant into their current state. After Yuya's initial discovery of Pendulum Monsters, Declan Akaba started a project within the Leo Corporation to create more Pendulum Monsters so that the entire world can have access to them. The first prototypes were unveiled in Declan's first duel with Yuya, but the summoning energy they gave off was unstable and so they malfunctioned. Also, these initial prototypes couldn't quite match Yuya's Pendulum Summon in terms of energy. Although they managed to stabilize their Pendulum Monster's energy in time for the Arc League Championship, it is was never directly stated whether or not the finished Pendulum Monsters were as good as Yuya's. When a Pendulum Monster is placed into a Pendulum Zone, it appears in a transparent column of blue light from a portal on the ground not dissimilar to the Summoning of monsters in Yu-Gi-Oh! 5D's.
In the anime, when a Duelist uses Pendulum Summoning to gather the materials required for a Fusion, Synchro, or Xyz Summon, it is referred to as performing a "Pendulum-Fusion/Synchro/Xyz Summon" respectively.
- So far, no Pendulum Monster has more than 3300 ATK and 3000 DEF.
- The "Qli" archetype currently includes all Scale 9 Pendulum Monsters.
- "Qliphort Scout" was the first Pendulum Monster to ever be on the Forbidden/Limited Lists.
- YGOrganization - [OCG] Pendulum Summons, New Mechanics, and Arc-V News!
-  - [OCG] Points of Interest from the New Rulebook
- yugioh-card.com/en Learn How to Pendulum Summon Tutorial Video (English)