How it plays
The game of Nines, by Nazmi Elyigit
These rules are Nazmi Elyigit's, in full. Nothing here has been simplified or house-ruled — where a choice was genuinely open, the app follows the original and says so.
The deck
Thirty-six cards: A through 9 in each suit. The ace is low. Four players in fixed seats, partners opposite, so the turn order alternates between the two sides.
The auction
Each player in turn either passes or names a higher number of tricks than the standing bid. Passing does not put you out — you are asked again on the next lap. The highest bidder becomes the declarer and commits their side to that many tricks.
Trump, then the reveal
The declarer names a trump suit, and only then does their partner lay all nine of their cards face up. That order is the whole difficulty: you commit on the nine cards you can see and find out about the other nine afterwards. The declarer plays both hands.
Playing a card
This is where the game differs from every other trick game. If you hold the led suit you must play it, and if you can beat the best card of that suit on the table you must. If you are void in the led suit you must trump, and if a trump is already down you must beat it. Only when you hold neither the led suit nor a trump may you play whatever you like — and then you cannot win the trick.
There is no exception for your partner. If the only card that beats the trick is yours and your partner is winning it, you take it off them.
Ending
The declaring side needs their contract; the defenders need the number of tricks that makes it unreachable. One of them usually gets there well before trick nine, and the hand plays on to trick three. A bid of 9 leaves the defenders needing exactly one trick.
What actually wins
Solved exactly over many thousands of deals, the features that predict tricks are not the ones card sense reaches for. A void is the most valuable thing on your hand, and a nine is next. Long suits are worth almost nothing. Both follow from compulsory ruffing: a void does not merely let you trump in, it guarantees you will.
The complete specification, including every rule variant, lives in RULES.md in the repository.