I learned this game from Ralf Brostedt's page, which includes rules and a free computer version of the game.
The name of the game means "20 down". It is played by four people with a 32-card pack, the cards in each suit ranking from high to low A-K-Q-J-10-9-8-7. Each player starts with a score of 20 and the object is to be the first to reach zero.
The game is played clockwise. The dealer deals two cards to each player, and the player to dealer's left must choose a trump suit on the basis of his two cards. The dealer then deals three more cards each so that everyone has five cards.
Beginning with the player to dealer's left, each player may discard up to three cards (all at once) and obtain an equal number of replacements from the dealer. A player does not need to discard any cards at all if he is satisfied with his initial hand.
Now players have the option of dropping out of the play, if they do not wish to risk the penalty for taking no tricks. Each player in turn decides whether to stay in or drop out. However:
- The player to dealer's left, who chose trumps, must stay in (so the player opposite dealer has the first choice).
- If diamonds are trumps, everyone must stay in.
The cards are played out in five tricks, the player to dealer's left leading to the first trick. Players must follow suit if they can, and a player who has no card of the suit led must play a trump if possible.
The scoring works as follows:
- Players deduct one point from their score for each trick that they win.
- A player who wins all the tricks subtracts five points instead of four.
- A player who stays in but takes no trick must add five points as a penalty.
- All the above scores are doubled if hearts are trumps.
- A player who drops out neither wins not loses anything.
If all players other than the trump maker drop out, then the trump maker wins all five tricks by default, and subtracts 5 points (10 if hearts are trumps).
If after eight deals have been played no one has reached zero, the rules change. The ninth and subsequent deals are played with hearts as trumps (so the player to dealer's left has no choice of trump suit). No one is allowed to drop out, and since hearts are trumps the scores are doubled.
The winner is the first player whose score becomes zero or less. If two or more players reach a score of zero or less on the same hand, the one with the largest negative score wins. If there is a tie for most negative score, further deals are played until there is a unique winner.
FestApp has published a 20-down Android app.