A Tag-Team Duel (also known as a 2v2 Duel) is a Duel played between two teams of 2 players each. All standard rules apply, and each team is given a shared LP counter of 8000 or 16000. TCG Official Tag Duel Rules as of August 27th, 2013.
When taking part in a Tag-Team Duel both teams consist of 2 players. Players share a combined life point total of 8000 and their fields. Neither player can look at/use their team-mates or their opponents hands or set/Faced-down cards. If one member of a team has been defeated that player's deck and field is unusable. They are not to be confused with the similar Battle Royal.
Alternatively, players can play in a style similar to Tag Dueling in various video games, such as the World Championship and Tag Force series. Both players share 5 Monster Card Zones, five Spell & Trap Card Zones, Graveyard, Banished Zone and Field Card Zone. The Duel proceeds like a normal Duel, with the Main Deck, Extra Deck, and hand changing as each teammate becomes the new turn player of the team at the start of every other turn. Any player-specific cards will only affect the turn players of each team at the moment.
Tag-Team in the anime
The first Tag Team Duel in the anime was Joey Wheeler and Yugi Muto vs. the Paradox Brothers. There have also been occurrences of 2 vs. 1 Duels (e.g Yugi and Kaiba vs. Dartz), 1 vs. 3 Duels (Tristan, Serenity and Duke vs. Nesbitt in the Virtual World Arc).
Tag-Team duels are also found in Yu-Gi-Oh! GX. The first was Jaden Yuki and Syrus Truesdale VS the Paradox Brothers, followed by Frost and Thunder VS Syrus Truesdale and Tyranno Hassleberry, then followed by Sarina clones VS Jaden Yuki and Aster Phoenix. A Tag-Team tournament was seen in Season 4.
The rules of the Frost/Thunder VS Syrus/Hassleberry duel are different than most Anime tag duels since the four players are given their own Life Point counter.
These rules have been also used in the Yu-Gi-Oh! 5D's series, when Crow Hogan and Yusei Fudo team up to face off against Sector Security. Furthermore, even though it took place through a Turbo Duel, the 2008 World Championship rules weren't changed. In the series it has been revealed that this type of Team Dueling is supported even on the world of Turbo Dueling. As there has been shown in the anime the existence of special Duel Runners for Tag-Team duels.
A 3-on-1 Tag Duel happened from episodes 62 to 64. Rex Goodwin, who was alone, started with 3 times the standard amount of Life Points, 12000. Yusei Fudo, Jack Atlas and Crow Hogan had separated fields, Decks and Life Points. Effects of cards used by Rex that targeted the opponent player targeted only one of the three, by his choice.
Also, in other instances according to the canonical movie 'Yu-Gi-Oh! 3D Bonds Beyond Time' during the 3 vs. 1 duel between Yugi, Yusei and Jaden vs Paradox, tag-team players are able to share cards on the field. An example of this is displayed when Yusei allowed Yugi to use Reincarnation of Hope's effect or when Jaden borrowed Junk Gardna to fuse with Elemental HERO Neos to summon out Elemental HERO Neos Knight.
In addition, Spells or Traps can be used to affect monsters that are under your partner's control.
Another one is seen in episode 37, but this time it was a 2v1 match, all players with separated Life Points, and if one player lost, his/her partner would lose as well. It is also seen at episode 42 where Yuma and Kite Dueled against Trey and Quattro.
Tag-Team specific cards
Some anime-only cards in the anime can only be used in a Tag Team Duel: