Two players, one 52-card deck, ten cards each. The next card is turned face up to start the discard pile, and the remaining thirty-one become the stock. The non-dealer plays first, and the deal alternates every round.

There is no trump suit, no bidding and no partner. Twenty-one of the fifty-two cards are visible or held at the start, which means most of the deck is still unknown — and the discard pile is the only place you ever get to watch it being revealed.

On your turn you take exactly one card and then let exactly one go, so your hand is eleven cards for a moment and ten again by the end. You may take the face-up card on the discard pile, or the top of the stock without seeing it first.

The one thing you may not do is take the face-up card and immediately put it back. That would be a free turn and a free look at nothing, so the rules forbid it — you have to keep the card you took until at least your next turn, and discard something else.

Taking from the pile tells your opponent something real, because they watched you do it. Taking from the stock tells them nothing. That is why a good player takes the face-up card only when it genuinely joins a meld, and otherwise draws blind.

A meld is either a set — three or four cards of the same rank, like three eights — or a run, three or more consecutive cards in a single suit, like the five, six and seven of clubs. A run does not wrap around: the ace sits below the two, so A-2-3 is a run and Q-K-A is not.

Everything not in a meld is deadwood, and deadwood is counted in points. The ace is one, the numbered cards are worth their number, and the jack, queen and king are ten each. So a hand full of low cards is a cheap hand to be caught with, and one full of court cards is expensive.

A card can only be in one meld at a time, which is where the thinking comes in. If you hold the seven of clubs alongside three other sevens and the eight and nine of clubs, you cannot use it twice — and putting it in the run rather than the set is what lets the other three sevens still meld together.

When your deadwood is ten points or fewer you may end the round by knocking. You knock as part of your discard: you draw as usual, then put your last card down and stop the round rather than passing the turn on.

Whether you may knock depends on the card you throw, not just on the hand you hold. Eleven cards with twenty points of deadwood might come down to exactly ten if you throw the king and nothing else, in which case the king is the only discard that can knock.

Knocking early is not obviously right. A thin knock — ten deadwood, on a turn when your opponent has barely drawn — can still win the round, but it gives them the chance to turn it around, which is what the next section is about.

Gin is a hand with no deadwood at all: all ten cards melded, nothing left over. It scores a flat 25-point bonus plus every point in your opponent's hand.

The bonus is not the only reason to want it. Going gin also takes away your opponent's right to lay off, so instead of shedding their loose cards onto your melds they are charged for all of them. Against a plain knock a hand with fifteen points of deadwood might pay four; against gin it pays fifteen.

Gin also cannot be undercut. An undercut needs the defender to finish at or below the knocker, and nothing is below zero, so a gin hand is the only knock that carries no risk at all.

When somebody knocks with deadwood left over, they lay their melds face up, and the defender gets to add their own loose cards to them. If the knocker put down the five, six and seven of diamonds, the defender's eight of diamonds goes onto the end of it and stops being deadwood. This is laying off, and it can chain: once the eight is down, the nine can follow.

Then the two deadwood counts are compared. If the defender is still higher, the knocker scores the difference. But if laying off has brought the defender level with the knocker or below them, the knock has backfired: the defender scores the difference instead, plus a 25-point bonus of their own. That is an undercut.

This is why a thin knock is a gamble. Knock on eight points against a defender holding sixteen and you expect eight points — but if eight of those sixteen slide onto your own melds, they finish level with you and take 25 instead. The melds you lay down are the ladder your opponent climbs.

Only one player scores in a round. A plain knock pays the knocker the difference between the two deadwood counts after laying off. Gin pays 25 plus the defender's whole hand. An undercut pays the defender the difference plus 25.

Because gin and the undercut both carry a 25-point bonus, a round is rarely worth nothing and occasionally worth fifty or more. Typical rounds land somewhere in the teens, and a match to 100 usually takes ten or twelve of them.

One round in twenty or so ends without a score at all. If the stock runs down to two cards and neither player has knocked, the hand is dead: nobody scores, and the cards are shuffled and dealt again with the deal passing over. It is not a bug, it is the game admitting that there is nothing left to draw.

Gin Rummy is one member of a large family, and the differences are smaller than the shared name suggests. This table plays standard Gin: knock at ten, gin worth 25, undercut worth 25, first to 100.

Oklahoma Gin is the same game with one rule changed, and it is a rule that changes everything. The card turned up at the start of the round sets the knocking limit for that round, so if a four is turned up you may not knock above four deadwood, and a turned-up ace means gin or nothing. Rounds get longer and thin knocks disappear.

500 Rummy is a different game wearing a similar hat. Melds go on the table face up as you make them, you can add to your opponent's melds as well as your own, the whole discard pile is available rather than just the top card, and you score for the cards you meld rather than only for the deadwood you avoid. It plays with more than two people and it rewards laying cards down early, where Gin rewards keeping everything hidden until the end.