How do you play Jigsaw Sudoku?

The rules: regions replace boxes

A jigsaw sudoku is a 9×9 grid divided into nine regions that interlock like jigsaw pieces — snaking, L-shaped, T-shaped, staircased — each containing exactly nine cells. The regions are printed with bold borders (apps usually add color), and your three constraints are:

• each row contains 1–9 exactly once,
• each column contains 1–9 exactly once,
• each region contains 1–9 exactly once.

That's it — no arithmetic, no diagonals, no extra digits. If you already play classic sudoku, you know the rules of jigsaw completely. What you don't yet have is the geometric intuition: years of classic solving train you to "see" in 3×3 blocks, and jigsaw quietly deletes that reflex. The first few puzzles feel oddly disorienting for exactly this reason — and it passes.

The Law of Leftovers — jigsaw's signature trick

Irregular regions unlock a deduction that doesn't exist in classic sudoku, usually called the Law of Leftovers. The idea is simple once you see it.

Take any block of complete rows — say the top two rows. Those 18 cells must contain the digits 1–9 exactly twice, because each row holds each digit once. Now look at the regions in that area. In classic sudoku, boxes align perfectly with every band of three rows, so nothing interesting happens. In jigsaw, regions spill over the line: part of some region pokes out below the boundary, and to make up the count, part of another region pokes in from below.

Here's the law: the cells poking out and the cells poking in must contain exactly the same digits. The rows account for a fixed multiset of digits, the regions inside account for most of it, and the leftovers on each side of the line have to balance. If a single cell sticks out and it's a 4, the single cell sticking in is also a 4 — a free placement from pure geometry, before you've applied any ordinary technique. The same argument works with columns, and with any block of one, two, three or four rows.

You don't need the Law of Leftovers on easy jigsaws, but on hard ones it's often the intended crack in the puzzle — and spotting a one-cell leftover feels like getting away with something.

How strategy changes from classic

Beyond the Law of Leftovers, everyday jigsaw strategy is classic strategy with the geometry dialed up:

• Region shapes create forced placements. A snaking region might cross five or six different rows and columns — far more than a 3×3 box's three and three. Each digit already placed in such a region eliminates candidates across a much wider swath of the grid, so region-based eliminations are stronger in jigsaw than box-based ones are in classic. When a region has one cell in some row, whatever that row still needs at that spot is forced.
• Box-line reduction gets sharper teeth. The classic move — "all of this box's 5s sit in one row, so no other 5 in that row" — applies to any region-line overlap, and irregular regions produce these overlaps constantly and in odd places. Scan for them deliberately.
• Cross-hatching still works, but slower. You can't sweep three aligned boxes at a glance anymore; each region needs individual attention. Expect your eyes to work harder in the opening phase.
• Pencil marks earn their keep earlier. Because the region geometry is unfamiliar, holding candidates in your head is less reliable than in classic. Marking candidates in the strangest-shaped regions first pays off.

Is jigsaw sudoku harder than classic?

At the same rated difficulty, jigsaw usually feels harder — not because the logic is deeper, but because none of your visual shortcuts apply. Classic solvers lean heavily on the fixed 3×3 geometry without realizing it; jigsaw takes that crutch away and makes you actually look at the regions.

Objectively, the picture is more interesting. Irregular regions constrain the grid in less uniform ways, and jigsaw puzzles can be constructed with fewer givens than classic ones while staying uniquely solvable — hard jigsaws can be genuinely mean. But an easy jigsaw is still an easy puzzle, and after a handful of grids the disorientation fades and your solving times drift back toward your classic pace. Most players land here: jigsaw is the cheapest way to make sudoku feel brand-new without learning a single new rule.

How to play Jigsaw Sudoku in Sudoku by Joseph — it's free

1. Install the app

   Download Sudoku by Joseph for iPhone, iPad or Android. Jigsaw is in the free tier alongside Classic and Killer — no purchase needed to play it.

2. Choose Jigsaw from the variant chooser

   Open the variant chooser and pick Jigsaw. The colored regions make the shapes easy to read at a glance — kinder on the eyes than bold borders on paper.

3. Pick your difficulty and solve

   Every difficulty from Easy to Diabolic is open from the first launch — no level map, no unlocks. Pencil marks are unlimited, hints are on request, and there is no mistake counter anywhere in the app.

Practical jigsaw tips

• Start each puzzle by tracing the region borders once with your eyes — thirty seconds of map-reading saves minutes of mis-scanning later.
• Hunt for regions that have exactly one cell in some row or column: whatever that line still needs there is forced. It's the most common jigsaw-specific placement.
• Check the top and bottom one or two rows for Law of Leftovers positions before diving into candidates — a one-cell leftover is a free digit.
• If you came from classic, resist scanning in 3×3 blocks. The old geometry is gone; deliberately scan region by region until the new shapes feel native.

Other options worth knowing

Honest alternatives: sudoku-puzzles.net offers free irregular sudoku to play in the browser or print, and PuzzleMix carries hand-made jigsaw puzzles from a respected UK setter if you enjoy puzzles with an author's fingerprint. Plenty of printable "squiggly sudoku" books exist too. Where Sudoku by Joseph differs is having jigsaw as a first-class, unlimited variant in the free tier of a native app — colored regions, every difficulty from Easy to Diabolic, offline play, saved progress, and no lives or coins anywhere.

Frequently asked questions