Doublecross is a card game for 4 players, played with a standard 52-card deck. Read the blog post on the game. These are the detailed instructions for playing; read bold print for the basics.
Players sit in a square, facing the center. Each player starts with all of the cards of one suit in his or her hand. Each player's Ace is placed directly in front of him or her, marking the player's suit for others to see, and not available for further play. The suits should be distributed alternating colors around the square (black red black red) such that each player is sitting next to players with the opposite color suit and across from the player with the same color suit.
The game proceeds in 12 turns. Each turn consists of a trading phase, in which players may trade a card, and a playing phase, in which each player plays a card against another player, which gives that other player points equal to the value of the card. Face cards are worth 10. The goal of the game is to finish with as few points played against you as possible.
During each trading phase, players may trade one card with the player sitting to their right OR with the player sitting to their left. Each player may trade only one card each turn, and never with the player sitting opposite them. Players may negotiate with either player sitting next to them for any reasonable amount of time. Players may also elect not to trade.
After trading, each player must play a card simultaneously. All players, on a count of "1, 2, *3*" will put down a card in a position that indicates which player it is being played against. E.g. if playing a card against the player across from oneself, place the card in front of the Ace that is laying in front of you (inert, marking your suit). To play against the player to your right, place the card to the front-right of your Ace.
If a player plays a card of a color opposite that of his or her suit against a player of that (opposite) color, it is worth double. That is, if a Spades (black) player plays a red card (Hearts or Diamonds) against a red player (one who started with Hearts or Diamonds), it counts double against that player. If a red player plays a black card against a black player, it is worth double. Otherwise put, cards that belong to either player sitting next to you are worth double when played against the player next to you. All cards count regular when played against the player sitting opposite you.
Cards that belong to the opposite suit (i.e. that of the same color of yours, belonging to the player across from you) may be used to "defend" rather than played against another player. Thus a Spades player can defend with Clubs (and vice-versa) and a Hearts player can defend with Diamonds (and vice-versa). These cards are played on top of the player's Ace and cancel any cards played against the defender of a value equal or lesser than double the value of the defending card. E.g. A defending "4" will cancel any card of 8 or below (a regular 8 or a doubled 4), a defending 7 will cancel an attack of 14 or below (such as doubled 7s, but would have no effect on doubled 8s or above), and a defending 10 would cancel all incoming cards that round. Each player may only defend himself or herself, and all cards that are used to defend or are defended against are discarded at the end of the turn.
Cards that are played against a player are collected in two piles. Cards that are played for regular value are kept in a pile to the left of each player's Ace, and cards that are played for double value are kept in a pile to the right of each player's Ace. If at any time a player wants to know another player's current score, it should be calculated and announced.
After 12 rounds, when all cards have been played, the points are calculated and the players are ranked in order of least points accrued. Another round may then begin.