An example Tridoku: nine triangular nonets (thick borders); the top one is filled 1–9 as an example. The amber cells run along the three outer edges, and the three grey half-shaded cells sit where an outer edge meets the inner triangle.
Tridoku — also called Triangular Sudoku — keeps Sudoku's 81 cells and its digits 1–9 but throws away the square. The board is one big triangle sliced into 81 small triangles that alternately point up and down, and those cells are grouped into nine triangular regions instead of nine square boxes. The digits are the same; the geometry, and the way cells "see" each other, is completely different.
The layout: a triangle of triangles
Picture a large triangle divided into nine smaller triangles of equal size — six pointing up, three pointing down. Each of those nine is a nonet, drawn with thick borders, and like a Sudoku box it must contain every digit 1–9 exactly once. In the diagram above the top nonet is filled in to show the idea; the other eight work the same way.
The four rules
- Nonets. Each of the nine triangular regions holds the digits 1–9 with no repeats — exactly like a Sudoku box.
- Outer edges. Each of the three sides of the big triangle is a line of nine cells that must also hold 1–9 with no repeats.
- Inner triangle. A second, shaded triangle sits inside, its corners at the midpoints of the three outer edges. Each of its three sides is likewise a line of nine cells with no repeats. The three cells where an outer edge meets this inner triangle (grey above) belong to both a side of the big triangle and a side of the inner one.
- Touching cells. No two cells that touch may hold the same digit. Because a small triangle can share an edge or a corner with many neighbours, one cell can "see" up to twelve others — the constraint that makes Tridoku feel so different from square Sudoku.
A worked approach, step by step
Step 1 — treat each nonet as a mini-Sudoku. Every triangular region still needs all nine digits, so scan for a region that already has several givens and fill the forced cells first.
Step 2 — use the edge lines like extra rows. The three outer edges and the three inner-triangle sides are nine-cell "no-repeat" lines. Where a line is nearly full, the missing digit often drops straight in.
Step 3 — lean on the touching rule. Before writing a digit, check every neighbour it touches — up to twelve cells. In tight clusters this rule alone eliminates most candidates and is where Tridoku's hardest deductions live.
Solving strategy
- Work the corners of the big triangle. The three corner cells each sit in a nonet and on two outer edges at once, so they are heavily constrained and a good place to start.
- Watch the three midpoint cells. Each cell where an outer edge meets the inner triangle carries two line-constraints at once — high-value cells for both placing and eliminating digits.
- Always re-check adjacency last. A placement that satisfies its nonet and lines can still break the touching rule; make that your final check before committing a digit.
Common mistakes
- Reading straight rows and columns. There are none — the only lines that matter are the three outer edges and the three inner-triangle sides.
- Undercounting neighbours. A triangular cell touches far more cells than a square one; miss a diagonal neighbour and the touching rule silently breaks.
- Forgetting the inner triangle. It is easy to solve the nonets and outer edges and overlook the shaded inner lines, which are a full constraint in their own right.
Can I play it here?
Not yet — Tridoku 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:
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.