How to Solve Jigsaw (Chaos) Sudoku

Chaos sudoku uses the same logic as classic sudoku, but the irregular regions mean some familiar methods no longer apply, and one new technique is added. This is how to approach it.

Start with what you already know

The classic techniques still work. Look for naked singles (a cell where only one digit is possible) and hidden singles (a digit that can only go in one cell of a row, column or region). Scanning rows and columns is unchanged — it's only the regions that have moved.

Read the regions from the colour

Because a region is now an irregular shape, follow the colour rather than a 3×3 box. Ask the usual question — "where can this digit go in this region?" — but trace the whole jigsaw shape, which may stretch across several rows and columns. A region that spans many rows restricts those rows more than a 3×3 box does.

The law of leftovers

This technique only exists in jigsaw sudoku, and it is worth learning. Take a band of three rows. Some regions sit entirely inside that band; others poke out of it. The digits in the cells that poke out must exactly match the digits in the band's cells that aren't covered by the fully-contained regions — the "leftovers." Both sets are the same size and contain the same numbers.

The most useful case is the smallest one: when a single region pokes just one cell out of the band, and only one cell of the band is left over, those two cells must hold the same digit. Finding these forced pairs often solves a large part of the puzzle.

Use pencil marks

Irregular regions make candidates harder to track in your head, so use pencil marks (notes). Filling in the possible digits for the tricky cells lets you spot pairs, triples and the leftovers pattern far more easily. The board has a pencil-mark tool for this.

Double chaos: solve both grids together

In double chaos, don't treat the two grids as separate puzzles. A digit forced in one grid is instantly true in the other, so switch between them: place what one layout gives you, copy it into the other grid, and see what the second layout's regions now allow. Progress alternates between the grids until the shared solution is complete.