Expert Sudoku Techniques
These nine are what is left when even the wings and chains come up empty. Five of them grow the X-Wing idea — bigger, then deliberately imperfect. Two argue from the fact that the puzzle has one answer rather than from the grid itself. The last plays two chains off against each other. Still no guessing.
- SwordfishAn X-Wing one size up: one shape confined to three columns across three rows, clearing all three columns.
- JellyfishThe same pattern at size four, and the largest that is ever worth hunting for by hand.
- Finned X-WingAn X-Wing with one cell too many. The extra cell ruins the rectangle everywhere except inside its own box.
- Finned SwordfishThe same repair applied to a Swordfish — far more common than the clean version, once you know to look.
- Finned JellyfishA four-line fish with a fin. The last of the fish family, and the one that turns up when nothing else will.
- WXYZ-WingAn XYZ-Wing with one more wing: four cells, four shapes, and one shape that cannot survive outside them.
- Unique RectangleThree corners holding the same two shapes. The fourth cannot join them, or the puzzle would have two answers.
- Avoidable RectangleThe same argument made with cells you solved yourself, which is why it is easy to walk straight past.
- Multi-ColouringTwo separate colouring chains for one shape, linked by a single sighting between them.
Coming from below? The hard techniques cover the X-Wing, the wings and simple colouring — every page here is built on one of those, and the fish pages assume you can already spot a plain X-Wing. Past these? The master techniques let a box stand in for a line, and add the almost locked sets, the long chains and the loops. If you would rather have the work done for you, the sudoku solver walks any grid one technique at a time and names each move.