|Appears in (sets)|
Gemini Monsters (known as Dual Monsters in the OCG) are a subtype of Effect Monster. They have effects, but these effects must be "unlocked" by performing an additional Normal Summon on them while they are face-up on the field (known in the OCG as a Second Summon). Until then, they are treated as Normal Monsters on the field and in the graveyard. You cannot special summon Gemini's from the hand or the deck by abilities that supports normal monsters, like "Ancient Rules", because they still count as Effect monsters while in the hand and Deck. However, since they count as Normal monsters while they are in the Graveyard, they benefit greatly from the many support cards that allow Normal Monsters to be summoned back from the Graveyard, like "Birthright".
All Gemini Monsters to date are treated as Normal Monsters while they are face-up on the field before being Gemini Summoned, or while they are in the Graveyard. If they exist anywhere else, they are treated as Effect Monsters.
The process of Gemini Summoning a Gemini Monster (that is being treated as a Normal Monster by its effect) is just the same as performing any other Normal Summon, except that the monster is already on the field and you don't have to Tribute monsters for Gemini Monsters of Level 5 or above. You cannot Gemini Summon a Gemini Monster that has already been Gemini Summoned (and is still considered to have been Gemini Summoned).
While this entire process seems very troublesome to go through for just one monster, note that Gemini monsters usually have extremely powerful effects, often without a drawback.
The Gemini Monsters were initially released in Tactical Evolution, with more released in Gladiator's Assault, Phantom Darkness, Light of Destruction, Structure Deck 17: Warrior's Strike and Stardust Overdrive.
So far, Gemini Monsters have yet to make a proper appearance in the anime or manga. However, several members have made brief appearances in Yu-Gi-Oh! 5D's and Yu-Gi-Oh! ZEXAL. "Phoenix Gearfried" is on the poster for the World Racing Grand Prix, "Gigaplant" appears in a flashback of Akiza Izinski's past, and "Crusader of Endymion" has been seen played by random duelists in Yu-Gi-Oh! 5D's - Episode 114 and Yu-Gi-Oh! ZEXAL - Episode 001. Also "Gemini" appears on signs in the background during several Turbo Duels in Yu-Gi-Oh! 5D's. Whether this was an intended reference to the archetype or not is unknown, since the archetype is not called Gemini in Japan.
Gemini monsters can play an important role in decks based around Normal Monsters. A good example of this is "Cu Chulainn the Awakened", seeing as he can use the attack points of a Normal Monster in the graveyard.
For quite some time, there have been a few Gemini Monsters that are part of an Archetype; the first being "Alien Hypno" and "Ancient Gear Knight" which were then followed by "Elemental Hero Neos Alius", and finally two Gem-Knight Geminis; "Amber" and "Iolite".
Decks built entirely around Gemini monsters are almost all Aggro decks with strong field control, some capacity for field-swarming, and quick synchro. However, the main strength of this archetype is versatility. Gemini decks can vary greatly in specialty and method of operation, so much so that (unlike many archetypes) any two examples could be almost entirely different outside of two or three monsters and traps. The major types of Gemini Decks are Field Control (which tend to focus on destruction effects), Equip (which focus on Equips and often lean towards Warrior Gemini monsters), and the most popular: Gigavise (which focuses on abuse of "Gigaplant's" effect to field-swarm Plants or Geminis, sometimes going for the Gigaplant OTK). Some may include elements of all these types. Now, with a debut of "Xyz Monsters", decks that revolves on Level 4 or lower Gemini and non-Gemini monsters has good potential in conducting a quick Xyz Summon of Rank 4 or lower Xyz Monsters.
Gemini monsters have seen an upswing recently in inclusion in anti-meta decks, with the powerful beatsticks ("Elemental Hero Neos Alius" and "Crusader of Endymion") splashing in with support from "Honest" and "Gemini Spark". The fusion monster "Superalloy Beast Raptinus" causes them to gain their effects immediately, and can be fused from any two gemini's.
Elemental Hero Neos Alius is an ideal Fusion Material monster for the "Omni Heroes" Elemental Heroes with non-specific fusion material based on the different attributes. What makes it really impressive is the use of "Super Polymerization", enabling not only discards (which are beneficial to a deck of this kind) but the theft of certain monsters depending on attribute.
All decks that deal with Geminis are extremely vulnerable to commonly played cards such as "Book of Moon" (which can reset the effects of Gemini Monsters) and "Bottomless Trap Hole" (which not only stops their summon, but removes them from play). Another problem with pure Gemini decks is speed. Even when built for field-swarming, Gemini decks can have a hard time matching the speed of the most commonly played archetypes that specialize in it, such as X-Sabers, Blackwings, and Lightsworns. Other weaknesses include Fortune Lady decks because of their Field Spell "Future Visions", which prevents Gemini Monsters from gaining their effects, and "Black Garden", which would cut down the ATK of a Gemini Summoned Monster to a quarter before it would gain its effects. Also, Gemini monsters tend to rely more heavily on support cards than some archetypes, so decks built around them more typically go over the 40 card standard than others.