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A Pendulum Monster (Japanese: ペンデュラムモンスター Pendyuramu Monsutā, 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. It can be Special Summoned by appropriate card effects. If it is also a Normal or Effect Monster, it can also be Normal Summoned/Set from the hand, just like any other Normal or Effect Monster.
While in a Pendulum Zone, a Pendulum Monster is treated as a Spell Card and not as a monster. A Pendulum Monster can be activated from the hand as a Spell Card in the player's Pendulum Zone, but it is not possible for Pendulum Monsters 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. As long as there is an available Pendulum Zone on the player's field, they can activate a Pendulum Monster from their hand in that zone.
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.
If 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 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 (including if it has its activation as a Spell Card or Summon negated). A face-up Pendulum Monster in the Extra Deck can be Pendulum Summoned, as long as its Level is between the Pendulum Scales.
Pendulum Monsters cannot be used to pay costs that send cards from the field to the Graveyard, as they would be placed in the Extra Deck instead; however, Pendulum Monsters on the field can be Tributed for costs. 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 being applied, this will override the Pendulum Monster's ability to go to the Extra Deck. 
Extra Deck Pendulum Monsters
While most Pendulum Monsters are also Normal or Effect Monsters (originating in the Main Deck), a small number of Pendulum Monsters are also Fusion, Synchro, or Xyz Monsters (originating face-down in the Extra Deck). Because they cannot be activated in the Pendulum Zone from the hand, they have effects to place themselves in the Pendulum Zone. Face-down Pendulum Monsters in the Extra Deck cannot be Pendulum Summoned, but can be Fusion, Synchro or Xyz Summoned (depending on their monster card type). Face-up Pendulum Monsters in the Extra Deck cannot be Fusion, Synchro or Xyz Summoned, but can be Pendulum Summoned.
As Xyz Monsters do not have Levels, Xyz Pendulum Monsters usually have a condition that allows them to be Pendulum Summoned from face-up in the Extra Deck if their original Rank matches a Level whose monsters their possessor can currently Pendulum Summon. Otherwise, they have a condition that prevents them from being Pendulum Summoned.
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. The upper bit of the Effect box (more or less corresponding to the Type line's height) is also transparent, but to a lesser degree than the Pendulum Effect box.
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 or Link Summon, as Tributes for a Tribute or Ritual Summon, or to activate multiple effects that activate when Summoned (possibly using SEGOC to prevent the important one of being negated).
Pendulum Monsters' main weakness is their hand advantage cost to place in the Pendulum Zone, causing them to be vulnerable to nuke effects as they will destroy the field and shut down Pendulum Decks until a Pendulum Scale can be restored. 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" or "Pendulum Reborn".
Cards that banish (like "Divine Knight Ishzark" and "Chaos Sorcerer"), specifically counter Spell Cards (like "Spell Canceller"), and deplete the Extra Deck (like "Zaborg the Mega Monarch" and "Extra Gate") are problems for Pendulum Decks. "Anti-Spell Fragrance" will also prevent Pendulum Monsters being placed in the Pendulum Zone at all, since cards cannot be Set in the Pendulum Zone. "Pot of Acquisitiveness", Spell & Trap removal (preferably Quick-Play, such as "Mystical Space Typhoon") and "Imperial Iron Wall" can deal with these cards, however.
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, Pendulum Monsters were created when Zarc and Ray Akaba's wills reacted to each other when the world was split into four during their duel. Yuya's Pendulum Monsters were originally Effect Monsters, but they were transformed by the power of Yuya's pendant into their current state when Zarc awakened for the first time. 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 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.
Leo Akaba later used Pendulum Monsters of his own, though it is unknown where, how, or when he obtained them.
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", "Pendulum-Synchro", or "Pendulum-Xyz Summon", respectively. A Pendulum Monster that is also a Fusion, Synchro, or Xyz Monster is referred to as a "Fusion-Pendulum", "Synchro-Pendulum", or "Xyz-Pendulum Monster", respectively.
- List of Normal Pendulum Monsters
- List of Tuner Pendulum Monsters
- List of Fusion Pendulum Monsters
- List of Synchro Pendulum Monsters
- List of Xyz Pendulum Monsters
- "Supreme King Z-ARC" has the highest ATK and DEF of all Pendulum Monsters, with 4000 ATK and 4000 DEF.
- "Performapal Momoncarpet" is the only Pendulum Flip monster.
- All Pendulum Spirit monsters in the game are from the "Martial Art Spirit" series.
- "Qliphort Scout" was the first Pendulum Monster to ever be on the Forbidden/Limited Lists.
- Unlike the other Monster Card types, there are no Pendulum Monsters that are not also members of at least one other Monster Card type.
- So far, there are no Ritual nor Link Pendulum Monsters.
- Coincidently, the card frame for both of these Monster Card types are blue-colored.
- This is the only Monster Card type that can exist in both the Main Deck and the Extra Deck.
- "The Organization | [OCG] Pendulum Summons, New Mechanics, and ARC-V News!". YGOrganization. February 17, 2014. Retrieved January 30, 2017.
- "The Organization | [OCG] Points of Interest from the New Rulebook". YGOrganization. March 6, 2014. Retrieved January 30, 2017.
- "Yu-Gi-Oh! TRADING CARD GAME". Konami. Retrieved January 30, 2017.