In 6 nimmt!, you want to score as few points as possible. To play the game, you shuffle the 104 number cards, lay out four cards face-up to start the four rows, then deal ten cards to each player. Each turn, players simultaneously choose and reveal a card from their hand, then add the cards to the rows, with cards being placed in ascending order based on their number; specifically, each card is placed in the row that ends with the highest number that's below the card's number. When the sixth card is placed in a row, the owner of that card claims the other five cards and the sixth card becomes the first card in a new row.
In addition to a number from 1 to 104, each card has a point value. After finishing ten rounds, players tally their score and see whether the game ends because someone has passed a particular point threshold. When this happens, the player with the fewest points wins!
X nimmt! starts with the same premise as 6 nimmt!, but changes these rules a few ways. Each player starts with eight cards in hand and a personal X row. Played cards will be placed into three rows, with those rows being able to hold three, four or five cards. Whenever someone takes cards from a row, those cards go into the player's hand, then the player must play one card into their personal X row.
All cards in this X row must also be played in ascending order! If you can't do this, then you place all of those X cards aside in order to start a new X row, with the value of those set-aside cards now being doubled.
A round ends after someone plays the final card in their hand, and whoever has the fewest negative points after two rounds wins.