Double Memory is a modified version of Concentration, a card game that is also known as Memory, or Shinkei-suijaku (神経衰弱) in Japanese. The game was created by Itsuki Sumeragi who played it against Yumeko Jabami.
The game starts with 20 chips, issued by the Student Council, that are individually worth 1 million yen for a total of 20 million yen.
The game, played between two players is similar to memory, albeit with slightly modified rules. The game requires both participants to find a match for a card they flip over in the 104 card game (2 decks). The cards must both match in terms of number and suit. If a player fails to find a match then it is the other players turn; if a player finds a match then their turn continues until they are unable to successfully find a match then their opponent may begin their turn.
The player that finds a majority of the 54 pairs of cards wins the game.
Itsuki reveals that she specially designed a deck of cards that produce a small image on the back that differentiates itself from the regular deck of cards her family's toy company produces. The cards are temperature sensitive and with the heat inducing circuit in her bag, she is able to keep the cards warm and retaining their secret image. When placed down the cards have a 2-3 minute cooling period which allows her to match the cards easily compared to her opponent and memorize the location of the rest of the cards if the cooling period is over.