Trumps in the Domino Game 42
As you probably know, 42 is a four-player domino game with bidding and trumps. It was "invented" in the 1800s to play a card-like game using dominos to circumvent the then moral ban on playing card games. The game 42 is easily learned, but some beginners have difficulty with the concept of trump dominos.What are trumps? In a deck of 52 cards, the four suits are spades (♠), hearts (♥), clubs (♣), and diamonds (♦). The numbers and faces on the 13 cards in each suit (ace through 10, Jack, Queen, and King) are the rankings of the cards. Aces are high but may be low (as in straights). When a suit is declared trumps, that suit is more powerful than the other three suits. When played, a trump card (or the highest trump played) will take any trick. (In the card game Spades, the trump suit is always spades. In the domino game 42, high bidder declares the trump suit.)
Forty-two (42) is played with a set of double-six dominos (28 tiles). There are seven domino suits, blanks through sixes, any of which may be declared the trump suit. Doubles may be declared a suit of their own, too. When a suit is declared trump, all seven dominos in that suit are trump dominos. For example, if fours are declared the trump suit, then all seven dominos with four pips (dots) on either end of the domino are trump dominos. The pips on the other end of the domino are the ranking of the domino in the trump suit. Doubles are high unless declared otherwise.
This is the part that some beginners have trouble understanding. The pip rankings of the trump suit have no relationship whatsoever with non-trump dominos with the same number of pips on either end. For example (fours the trump suit), the four-deuce and the deuce-ace are from different suits. If the deuce-ace 
On an opening lead, if the high bidder does not declare a trump suit, the high end of the domino led is the suit to follow. For example, if he leads the five-four, fives is the suit led, and each player has to play a five if he has one. On the other hand, however, he can call either end the trump suit. If he calls fours trumps and leads the five-four, then each player has to play a four if he has one. In this case, the four end is the trump suit, and the five end is the ranking of the domino in the trump suit.
Finally, trump dominos may be played only when trumps are led or when a player cannot follow the suit led. If a player cannot follow a non-trump lead, he does not have to play a trump if he has one. High bidder does not have to lead a trump in the opening play, and a player can play any domino if he cannot follow the suit led. Knowing when to play your trumps is part of the strategy in making bids and setting opponents in the game of 42.
More on trumps: Instructions Q&A #58 Q&A #118
