updn is a trick-taking game using domino-style cards with two values, where either number can be the rank and either number the suit.
It is a "must follow" game, where you must play a card that matches the suit of the lead card if you can. However, the card with the highest sum of its two values will win the trick, regardless of suit. The winner will take all of the cards played to the trick, with some cards worth positive points and some cards worth negative points.
The game also uses a row of bonus cards to effect certain conditions, with the bonus cards taking effect every two tricks (2, 4, 6, 8, but also 9). These cards do things such as reverse rankings (so that low sum wins), and are also worth points themselves. The card which is available in round 9 will reverse the player's score - making a score of -x worth +x, and vice versa.
The player with the most points wins.