Use a standard 52-card deck. King is high; Ace is low.
NOTE: An Ace is always low card in Gin Rummy; it cannot be used as high card. Also, face cards are worth 10 points each; number cards are worth their face value; an Ace is worth one point.
Collect sets (three or four of a kind, or three or more consecutive cards of the same suit) to earn points. The game is played over several rounds.
Choose a dealer randomly to deal the first round; throughout the game, the winner of each round deals the next.
Shuffle the deck and deal 10 cards to each player. Players should look at and sort their cards.
The next card is turned face up in the middle of the table to start the discard pile. The remaining cards are placed face down next to the discard pile to form a draw pile.
Each normal turn consists of two parts.
First, you must take a card -- either the top card from the draw pile or the top card from the discard pile.
Second, you must discard a card (face up) onto the top of the discard pile.
NOTE: On the very first turn of each round, the non-dealer decides whether or not to take the first face up card. If that player declines, the dealer may take the card. If one of the players takes the card, that player completes his turn by discarding and then the other player takes a turn. If both players decline to take the card, the non-dealer starts the game by drawing the top card from the draw pile.
The round ends when a player "knocks." This may be done on any turn (including the first turn) after drawing but before discarding. A player may knock when he has the ability to form sets, discard one card, and have 10 points or fewer remaining in his hand.
NOTE: A single card cannot belong to two sets.
After knocking and discarding, the player who knocked organizes and spreads all of his cards face up on the table.
The player who did not knock does the same. If the knocker did not go gin (see "Going Gin" below), the opponent is also allowed to lay off any unmatched cards by adding to the knocker's sets (e.g. adding a fourth card to a group of three of a kind, or adding further consecutive cards of the same suit to a sequence).
NOTE: You're never required to knock. You may continue playing in an effort to develop a better hand.
Each player calculates the value of their unmatched cards. If the knocker's count is lower, he scores the difference between the two counts.
If the knocker did not go gin, and the values are equal -- or the knocker's value is greater than his opponent's -- then the knocker has been undercut. The knocker's opponent scores 10 points plus the difference between the values.
If the knocker has no unmatched cards, it's known as "going gin" and he scores 25 bonus points (some sources say the bonus should be 20 points). Additionally, his opponent cannot score any points, even if his opponent also had no unmatched cards.
If only two cards remain in the draw pile after a player discards and neither player has knocked, the round ends in a draw. The same player deals again.
Additional rounds are played until one player's cumulative score reaches 100 points or more. That player is the winner.