All Sudoku Variants
9×9 · Irregular Region

Chaos Construction

Also known as Chaos Construction Sudoku, Region Construction

The nine regions are not drawn in — you deduce their boundaries as you solve, so that each region is nine orthogonally-connected cells and every row, column and region still holds 1–9 once.

5 7 4 8 6 4 6 5 8 9

An example Chaos Construction puzzle: the nine regions are not drawn — deducing where each one lies is part of the solve.

Chaos Construction is a jigsaw Sudoku with the boundaries rubbed out. You still fill a 9×9 grid so every row, column and region holds 1–9 exactly once — but the nine regions are not drawn for you. Working out where each region begins and ends is part of the puzzle, done in step with placing the digits.

The rule: build the regions as you solve

Every region must be a block of nine orthogonally-connected cells (touching edge-to-edge, never only at a corner), and the nine regions must tile the grid with no overlaps or gaps. On top of that stands the ordinary Sudoku law: no digit repeats in a row, a column or a region. Because the regions are unknown, digits and boundaries constrain each other — a region can never contain two of the same digit, so two identical digits sitting side by side must fall in different regions.

4 7 2 2 1 5 9 6 8

A fragment: the two 2s (r1c1's neighbours) must land in different regions, which pushes the boundary between the blue and yellow groups. In Chaos Construction the shape of a region is itself a deduction.

A worked example, step by step

Step 1 — anchor with repeated digits. Two equal digits that are adjacent, or that would share a nine-cell block, cannot be in the same region. Each such pair forces a boundary between them, giving you your first region walls.

Step 2 — grow from the corners. A corner cell has few neighbours, so the region containing it has limited ways to reach nine connected cells. Trace those routes and the impossible ones fall away.

Step 3 — let digits and shapes feed each other. Once a region is pinned down, it becomes an ordinary Sudoku unit — scan it for singles. Each new digit, in turn, rules out region shapes that would repeat it. The two deductions leapfrog until the grid closes.

Solving strategy

  • Hunt for clashing digits first. Identical digits that are close together are your richest source of forced boundaries.
  • Count connectivity. A region needs exactly nine linked cells; a pocket that can only ever reach eight tells you a wall must open somewhere.
  • Treat a finished region as a normal box. The moment its nine cells are fixed, standard Sudoku scanning applies inside it.
  • Keep both layers in view. A placement you make for digit reasons may break a region, and vice versa — check both before committing.

Common mistakes

  • Diagonal "connections". Cells joined only at a corner are not connected; a region must link edge-to-edge throughout.
  • Forgetting the count of nine. Every region is exactly nine cells — no more, no fewer — so a stranded group of the wrong size signals an error.
  • Fixing regions too early. Commit a boundary only when it is forced; a guessed wall propagates fast and is painful to unwind.

Can I play it here?

Not yet — Chaos Construction isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile: