1 Deck. Hard (6%). Skill/Luck balanced.
Object
To discard all the cards in pairs.
Layout
•  15 (or 12 in 640x480 resolution) Piles arranged in a 3x5 rectangle discard pairs of cards that are one different from each other (Aces and Kings are considered consecutive) and are adjacent to each other, either side by side or diagonally. Spaces are filled as follows: any card above a space slides down to fill the space, empty spaces at the top are filled by the . 
•  (Face down, middle)  automatically fills spaces at the top of the layout. 
•  Discards (right)  all cards paired up and discarded from the layout are piled in 13 piles by , for easy counting of remaining cards. 
Notes
•  Discard pairs of cards that are one different from each other (for example, a Two may be paired with either an Ace or a Three) and are adjacent to each other, either side by side or diagonally. Kings may be paired with Aces. To discard a pair, drop one of the cards on the other, or rightclick on one of the cards. Spaces are filled as follows: any card above a space slides down to fill the space, empty spaces at the top are filled by the . This is the same way that pieces fall in the game Tetris. 
•  Discarded pairs are moved to 13 piles to the right of the , grouped by . This makes it easy to see what cards remain to be paired. This is very important as it is very easy to make moves that will make it impossible to win. For example, if you pair all the Tens with Nines and all the Queens with Kings, the Jacks have nowhere to go and can never be paired. 
History
Tetsol is based on an earlier game of the same name in the 16bit Pretty Good Solitaire. That game had a different pairing method and was invented by Randy Rasa. This game keeps the same layout, but changes the pairing from cards that add to 13 to pairs of cards one different. This makes for a more strategic game.
