The goal
Fill every square so each row, each column and each 3×3 box holds the digits 1 to 9 exactly once. Every puzzle has exactly one solution and can be solved by reasoning alone.
Two colors of notes
Notes sit in a small 3×3 grid inside each square: 1 top-left, 5 in the center, 9 bottom-right. Use gray Notes for candidates and orange Notes 2 for anything you want to stand out — pairs, a chain you are testing, or a guess. Placing a digit clears it from the notes of every square it sees.
Mistake check
With Mistake check on, a wrong digit turns red right away and counts as a mistake; the third mistake ends the game. Turn it off to play blind: nothing is marked until the board is full, then the whole board is checked and you can keep fixing it or reveal the wrong squares.
Auto-finish
When every empty square has only one possible digit left, the board fills itself in. Turn Auto-finish off if you'd rather place every last digit yourself.
Seven levels
Easy and Medium need only singles. Hard adds pointing and pairs, Expert X-Wings and hidden pairs, Master wings and swordfish, Extreme chains. Nightmare needs several forcing chains — assume a digit and follow it to a contradiction. The first digit of a game number is its level; the same number always deals the same board.
Keyboard
Arrows move. 1–9 fill in (or note, in a notes mode). Shift+digit toggles a gray note, Alt/Option+digit an orange one. Backspace, Delete or 0 erase. N turns Notes on or off, S does the same for Notes 2. Z undoes, P pauses.
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