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Game mechanics refer to any action that automatically happens due to the interior workings of the Yu-Gi-Oh! game. They are not treated as effects and most of the times cannot be avoided.
Every card has a specific name, but even something as simple as a name has rules attached to it. The basic ruling is that each deck may only contain up to 3 cards of the same name, but there are some cards with effects that change their name, like the Harpies. "Cyber Harpie Lady," "Harpie Lady 1," "Harpie Lady 2," and "Harpie Lady 3" all have the condition "(This card's name is always treated as "Harpie Lady".)" This means a Deck can only contain up to 3 copies of cards that are named or always treated as "Harpie Lady" (in any combination). Cards that change their name only at certain times such as "The Lady in Wight" or "Harpie Queen" are different and are not held by the same restriction.
A misconception among new players is what happens when considering that a name is in-play. If the name of a card is considered to be on the field, the effects of that card are not applied. While "Gravekeeper's Priestess" is on the field, "Necrovalley" is considered to be in-play, but while this allows you to use "Royal Tribute", that doesn't mean that "Necrovalley's" power to prevent cards in the Graveyard from being banished is active. Your opponent can still Special Summon "Chaos Sorcerer" from the Hand.
Most cards have their names changed from the OCG to the TCG for one reason or another, though normally harmless, it sometimes causes confusion or problems between archetypes; these card are often renamed (such as "Oscillo Hero #2" was to "Wattkid") or given an archetype condition to clarify this.
Many cards have the effect of creating Tokens for the field. Every Token Monster's original ATK, DEF, Level, Attribute, and Type are as they are listed on the card that summoned them. They can also have effects, but while they are considered written on the Token Monster itself, they are always considered Normal Monsters.
Token Monsters have a few rulings that go with them. First of all, they cannot be face-down on the field, so "Book of Moon" cannot Target them. If an effect that would flip multiple monsters on the field to face-down Defense Position effect them, they are simply switched to face-up Defense Position. Secondly, they cannot exist outside the field. If a card effect would remove them from the field (by sending them to the Hand, Deck, Graveyard, etc.) they simply disappear. If they are Destroyed by Battle they never hit the Graveyard, so "Blackwing - Shura the Blue Flame" wouldn't be able to activate its effect. Finally, Token Monsters are always owned by the owner of the card that summoned them. Thus, if "Owner's Seal" is activated, "Ojama Tokens" move to the side of the field of the player who activated "Ojama Trio". Additionally, Token Monsters cannot be used for Xyz Summons.
As long as the card that summoned them allows it, Token Monsters can be used as Tributes for Tribute or Ritual Summons, or as Synchro or Fusion Material Monsters. However, if a card requires the Tributed monster to go to a certain zone outside the field, like the Graveyard or Banished Zone, Token Monsters cannot be used.
Using Cards and the Graveyard
Whenever a card is Destroyed or Discarded, when a Normal or Quick-Play Spell is used, or when a Normal or Counter Trap is used, it is always sent to the Graveyard unless a card effect states otherwise, such as "Macro Cosmos" and "Masked HERO Dark Law".
Field Spell Cards
Only one Field Spell Card can be on the field at a time. If a player has one on the field and the other player Activates a second one, when the second one's Activation resolves, the first one is automatically destroyed. Because this is a Game Mechanic and not a card effect, cards like "Magical Citadel of Endymion" are not protected from this destruction. However, if the second Field Spell Card is destroyed before its activation resolves such as with "Mystical Space Typhoon", the original one is not destroyed.
The player who controls the Field Spell Card is also allowed to play a different one. In this situation, however, the first one is destroyed and sent to the Graveyard before the second one is activated. This creates a minor gap of time in which there is no Field Spell Card on the field, a large enough gap to destroy cards who cannot survive without the presence of one, such as the "Earthbound Immortal" and "Malefic" monsters.
Due to the 2014 rule change, both players are allowed to control one Field Spell Card at the same time.
Equip cards continuously target the equipped monster to make sure it is still there. If that monster is no longer face-up on the field, the Equip card is automatically destroyed. This can be due to either being destroyed or removed from the field by battle of card effect, or simply being flipped face-down with "Book of Moon".
Additionally, Equip cards also continuously target to make sure the equipped monster fits its requirements, and if it suddenly no longer meets its requirements, the Equip card is destroyed.
- Example: If "Machina Peacekeeper" is equipped to a "Machina Fortress" and "Zombie World" is Activated, "Machina Peacekeeper" is automatically destroyed because "Machina Fortress" has been changed into a Zombie-Type and no longer meets the Machine-Type requirement.
There are multiple cards with the following effect: "You can substitute this card for any 1 Fusion Material Monster. When you do this, the other Fusion Material Monster(s) must be the correct one(s)." While these are great cards for Fusion Summoning, there are other limits to their effects that aren't listed.
They cannot be used for the Special Summon of an "Elemental HERO Neos", "Gladiator Beast", or "A-to-Z" Fusion Monster, because the listed monsters are not used as Fusion Material Monsters. They can only be used as substitutes for specifically named Fusion Material Monsters, thus they cannot be used as any "1 Ojama Monster" for "Ojama Knight" And while they have effects while in the Hand, they have no effects while in the Deck, so they cannot be used when fusing from the Deck with cards like "Future Fusion".
Whenever a Pendulum monster on the field in either a Monster Zone or a Pendulum Zone would be sent to the graveyard, it is placed on the Extra Deck, face-up, and due to that part of their mechanic, they cannot be sent to the Graveyard from the field to pay a cost. While there, it is treated as an additional card on the Extra Deck, rather than a card in a card zone, so it cannot be targeted by card effects.
Within the TCG there are cards that, when combined together, create loops that can (and just might) never end. As such, a rule has been put in place for both types of infinite loop. First is the No Net Change loop. This type of loop is one where the card effects continue to loop around in a circle without any real changes or control of whether they happen or not. Players may not activate or summon a card if they know it would create a No Net Change loop. If this has been done by accident, play rewinds to the point of the offending activation or summon and continues from there.
Additionally, there are Net Change loops are those that also loop endlessly without any control, but a change is being applied with each loop, such as a loss of Life Points or the number of cards in a player's deck being reduced. Loops such as these are legal and are allowed to loop endlessly until the change stops happening, in which the card causing the loop will be automatically destroyed at the end of a loop. A loop where the only Net Change is the increase of a player's Life Points is considered a No Net Change, because a game cannot be ended due to a player having too many Life Points.
All other loops, such as the Quillbolt Loop, are controlled by a player and can be stopped at anytime when the player sees fit. Such loops are completely legal, but if it becomes a controlled No Net Change loop, the player cannot perform it; this would be a form of stalling and is not legal. An example would be a player using a Burn Quillbolt Loop when the opponent has Activated "Hallowed Life Barrier" for the turn.