The Side Deck is a group of 0-15 cards that you keep separate from the Main Deck. During a Match, you can interchange the cards between the Side Deck, Main Deck, and Extra Deck. As most sponsored tournaments are played in Match Games, the Side Deck often plays a large role in the outcome of the tournament, with skill-full siding often allowing for the victory of the whole match.
 Types of Siding
Although these are categorized as different types of siding, they are often used together in the side.
The most common use of the side, this type of siding is where the deck uses cards in the side that are counters to possible problem matchups. For optimum use competitively, it is generally a good idea for the sided cards to have a general usage for various decks. For example, a card that specifically destroys Fiend-Type monsters to counter Dark World specifically should be avoided in favor of generic cards which hurt the deck as well as several others, such as Dimensional Fissure.
As its name suggest, using the side as a "Toolbox" is to place various other options for the deck in the side to swap out between matchups. An example of this commonly performed in the current Metagame is a single sided copy of Bottomless Trap Hole and Dimensional Fissure, which are to be sided in against matchups where they are effective, with ineffective cards to be sided out.
Often performed by the King Deck#Metagame of the format, Counter Siding is used when the player knows that the majority of his matchups will side in cards for the player's deck. Therefore, the player adds cards to his side which counter the cards sided in by the opponent in response to the player's deck. Counter Siding is also often performed when the player knows that certain generic "staples" may be sided in to counter his deck, such as Macro Cosmos or Dimensional Fissure. A common tactic involving counter siding for players who are confident in their deck's Game 1 matchups is to make their main deck as consistent and accelerated as possible (occasionally through the use of cards such as Upstart Goblin, and to side out the drawpower in favor of more defensive options Game 2 in preperation for the opponent's side deck.
 Deck Changing
A clever and viable application of the Side Deck, many players can completely change to a different deck through the use of their Side Deck. However, this is difficult to perform, and costs the player of an actual Side Deck to deal with problem matchups.
 Commonly Sided Cards
Although the cards commonly sided vary between each Format, there are a few general criteria for the side. Cards sided in generally have to be:
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