What suit is that domino?
Rules
A playing card has one suit. A domino has two ends, so it belongs to two suits at once. The six-trey is a six and a trey. That is the source of most arguments about following suit, so here is the rule, slow.
Suit means a number, blank through six. Every domino with that number on either end is in that suit. The fours are the double-four, six-four, five-four, four-trey, four-deuce, four-ace and four-blank. Seven dominoes to a suit.
Trump is the suit the high bidder names. Once a suit is trump, every domino with that number on it is a trump and nothing else. If fours are trump, the six-four is a trump. It is no longer a six.
Following suit means playing a domino of the suit that was led. If you can, you must. If you cannot, you may play anything, including a trump.
So what suit was led? Two cases.
1. A trump was led. If the domino led has the trump number on either end, trump was led. You must play a trump if you have one. 2. Anything else was led. The suit is the higher end of the domino. The six-trey is led as a six. The five-ace is led as a five. A double is led as its own number.
Then, to follow, you need a domino with that number on it that is not a trump.
An example. Treys are trump. Somebody leads the five-deuce. That is a five, because five is the higher end. You must play a five if you have one. Your deuce-ace does not count. It has a deuce, not a five. Your five-trey does not count either. It has a five on it, but treys are trump, so it is a trump and not a five.
If you have the five-ace, you have to play it. If you have no plain fives at all, then you can play anything you like, and that is when the five-trey could go on as a trump.
Which domino wins? If anybody played a trump, the highest trump wins. If nobody did, the highest domino of the suit led wins. Within a suit the double is highest, then it goes by the other end. In sixes that is the double-six, then six-five, six-four, and on down to six-blank. A domino of some other suit, played because you could not follow, cannot win.
Playing off-suit when you could have followed is called a renege. What happens when one is caught is something every table should settle before the game starts, so nobody is making it up mid-argument.