How to Solve Jigsaw (Chaos) Sudoku

Chaos sudoku rewards the same logical thinking as the classic game, but the irregular regions mean some familiar habits don't transfer — and one powerful new technique appears. Here's 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, train your eye to 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. Regions that bend through many rows constrain those rows in ways a tidy box never could.

The law of leftovers

This is the technique unique to jigsaw sudoku, and it's 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. Spotting these forced pairs often breaks a puzzle wide open.

Use pencil marks

Irregular regions make candidates harder to track in your head, so lean on 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's pencil-mark tool is there for exactly 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 bounce between them: place what one layout gives you, carry it across, and see what the second layout's regions now reveal. Progress alternates between the grids until the shared solution falls out.