Rules of Texas 42

Texas 42 is a trick-taking game played with dominoes instead of cards. Four players sit in two partnerships, facing their partner across the table, and bid for the right to name trump and try to win at least as many points as they bid.

The dominoes

A standard double-six domino set has 28 tiles: every pairing of two numbers from 0 (blank) to 6, including each number paired with itself (the doubles). Every tile belongs to one or two "suits" -- the numbers on its two ends. A double belongs only to its own suit; for example 4-4 is a suit-4 domino and nothing else.

Five dominoes carry bonus points, called count, on top of the point each trick is worth:

Domino	Value
5-5 and 6-4	10 points each
5-0, 4-1, and 3-2	5 points each

That's 35 count points, plus 1 point for each of the 7 tricks in a hand -- 42 points total, which is where the game gets its name.

Dealing

The dominoes are shuffled and each of the four players is dealt 7. The deal rotates to the left after every hand.

Bidding

Starting with the player to the dealer's left, each player in turn either bids or passes -- there's only one round, so once you pass you're out of the bidding for that hand. A bid must beat the current high bid. Bids run from 30 up to 42 points; a player can also bid in marks (see below), which always outranks a plain point bid. If the first three players all pass, the dealer is stuck with it and must bid -- the dealer can't pass, so every hand gets played.

How this app bids: the dealer bids last, same as always, but is restricted to raising the bid by exactly one point (or jumping straight to a marks bid) rather than free-raising like the other three seats. Any player -- not just the dealer -- can open a marks bid, and once a marks bid is on the table, the only way to outbid it is to go exactly one mark higher.

Marks

A mark is the unit the match is scored in. A plain bid of 30-42 is worth 1 mark if made. A player can instead bid a fixed number of marks -- 2 marks or more -- which is a much bigger claim: it means committing to win every single trick in the hand. Pull it off and you score that many marks; fall one trick short and the opposing team scores that many marks instead. Because of how much it swings the match, a marks bid is usually reserved for a hand that's overwhelmingly strong in one suit.

Naming trump

Whoever wins the bidding names one of the seven suits (0 through 6) as trump. Every domino containing that number becomes trump for the rest of the hand and outranks every domino of every other suit. Within trump -- and within any suit -- the double is the highest domino, then it ranks down by its other number: for example if 4 is trump, the order from strongest to weakest is 4-4, 6-4, 5-4, 4-3, 4-2, 4-1, 4-0.

Instead of naming a suit, the bid winner can declare No Trump (sometimes called "Follow Me"). No domino is trump at all -- whichever suit gets led, the highest domino of that suit simply wins, and nothing can be played in to override it. This makes doubles especially strong, since a double is already the top domino of its own suit and now has nothing that could beat it either.

Playing a trick

The bid winner leads the first trick with any domino they choose. Whichever suit that domino represents is the suit "led" for the trick:

A trump domino always leads trump, regardless of its other number.
A non-trump double leads its own suit.
A non-trump, non-double domino leads whichever of its two numbers is higher.

Every other player must follow with a domino of the suit led if they have one. A domino that contains the trump number is trump only -- it doesn't count as belonging to its other number for follow-suit purposes. If you have nothing of the suit led, you may play anything, including a trump to try to win the trick, or a safe throwaway. Under a No Trump contract there's no first bullet to worry about -- nothing is ever trump, so every lead is just a double leading its own suit or a non-double leading its higher number, and following suit works exactly the same way.

The trick is won by the highest trump played, or if no trump was played, the highest domino of the suit that was led. The winner collects the trick and leads the next one. After 7 tricks the hand is over.

Scoring a hand

Add up the tricks and count each team collected. If the bidding team's total meets or beats their bid, they score the marks that bid was worth (1 for a plain bid, more for a marks bid) and the other team scores nothing for that hand. If the bidding team falls short, they're set: they score nothing, and the opposing team scores those marks instead.

Ending a hand early: this app stops play the moment the outcome is no longer in doubt -- either the bidding team has mathematically locked up their bid, or they've lost a trick they needed for a marks contract -- instead of always dealing out all 7 tricks.

Winning the game

The first partnership to reach 7 marks wins the match.