One of the ways you win at Gin Rummy is to knock before your opponent does. The question then becomes, how do you know the amount of points you have to be reduced to before you knock?
Besides the regular standard 52 card deck, you also use what is known as the knock deck. The knock deck is also another standard 52 set of playing cards that should be a different color or style from the regular deck that you use to actually play the game.
The knock deck is well shuffled before each new game, not each individual hand. It is placed face down into either the middle of the table next to the standard deck that you are using to play the game, or into the card case that you use for both of the decks. The top card of the deck is turned face up and placed against the second card so that it remains exposed for the entire game. This is the knock card for the first hand.
It represents the minimum knock for that particular hand. For example, if you turn over a 6 then the minimum amount of points you can have before you knock is 6. If you turn over a J, Q, or K the knock amount is 10. If you happen to turn up an Ace then you are not allowed to knock at all, the hand must be played completely for Gin. Whether it is a 2, 4, or 6 player game each of these teams are bound by this card and cannot legally knock without having less than that number of points.
At the end of the hand, the present knock card that is facing up is removed and placed at the bottom of the deck face to face with the bottom card. The second card in the deck is now turned face up and placed back to back with the next card. This is the knock card for the second hand. At the conclusion of the next hand, that card is then removed and placed directly behind the previous knock card facing up into the deck. This process continues throughout the whole game, with each hand getting a new knock card and subsequently a new amount of points everyone can knock with.
The positioning of the used knock cards allows any player to verify the last knock card as well as how many knock cards have already been used. This is mostly done in the case of a dispute, but you can check the knock cards that have been used at any point in the game. If by any chance the previous knock card was not changed for a new hand, that card will remain the knock for the new hand if anyone has discarded already without making that discovery. This doesn’t happen very often, but when it does it is best just to continue on instead of potentially changing the outcome of the hand. At the end of the game, not the individual hand, the knock deck is reshuffled and used for the next game.
If a knock deck is not available the knock is established in a different way. Each player is dealt 10 cards face down as normal, but instead of the dealer giving the 21st card face down to his opponent, he turns the card face up. This open card establishes the knock but its face or pip value.