Play Kuromasu Online
Place black cells so each numbered clue sees the correct count of white cells in all four directions. No two black cells may touch, and all white cells must stay connected.
Tap a cell to shade it black — tap again to clear
What Is Kuromasu?
Kuromasu (くろますー), also known as Where is Black Cells? or simply Kurodoko, is a binary-determination logic puzzle first published by the Japanese puzzle company Nikoli. The name roughly translates to “black cells” in Japanese. Like many Nikoli puzzles, Kuromasu combines simple rules with deep logical reasoning, making it a favourite among pencil puzzle enthusiasts worldwide.
The puzzle is played on a rectangular grid. Some cells contain a number. Your task is to determine which of the remaining cells should be black (shaded) and which should remain white (unshaded). The numbered clues tell you how many white cells are visible from that position, looking in all four orthogonal directions — up, down, left, and right — including the clue cell itself. Black cells block the line of sight.
Rules of Kuromasu
- Numbered clues are always white: A cell containing a number can never be shaded black. The number indicates the total count of white cells visible from that cell in all four orthogonal directions (up, down, left, right), counting the cell itself.
- Black cells block visibility: When counting visible white cells from a numbered clue, you stop counting in a direction as soon as you hit a black cell or the edge of the grid.
- No adjacent black cells: No two black cells may be horizontally or vertically adjacent. Diagonal adjacency is allowed.
- White cells stay connected: All white (unshaded) cells must form a single orthogonally connected group — you can reach any white cell from any other by moving through white cells horizontally or vertically.
- Unique solution: A well-formed Kuromasu puzzle has exactly one solution, reachable through pure logic with no guessing required.
How to Solve Kuromasu Puzzles
1. High Numbers Mean Open Space
A cell with a large number (close to the maximum possible visibility) needs white cells extending far in multiple directions. The cells near it are almost certainly white. For instance, on a 7×7 grid a clue of 13 (the maximum) means the entire row and column through that cell must be white.
2. Low Numbers Force Nearby Black Cells
A clue of 1 means only the clue cell itself is visible — all four of its orthogonal neighbours must be black (or the grid edge). A clue of 2 means only one additional white cell is visible in one direction, so black cells must block the other three directions quickly.
3. Use the Adjacency Rule
Since no two black cells may touch orthogonally, placing a black cell forces its four neighbours to be white. This cascading constraint is key to making progress — every black cell you place reveals safe white cells around it.
4. Maintain White Connectivity
All white cells must form one connected group. Before shading a cell black, check whether doing so would split the white region into disconnected parts. Corner and edge cells are especially sensitive to this rule.
5. Count and Compare Visibility
For each numbered clue, count the current visible white cells in all four directions. If the count already matches the number, place black cells at the boundaries. If placing a black cell in a direction would reduce the count below the target, that direction must remain open.
6. Cross-Reference Clues
When two numbered cells share a row or column, their visibility constraints interact. A black cell placed to satisfy one clue may force or prevent black cells needed by another. Look for these overlapping constraints to make advanced deductions.
Grid Sizes & Difficulty Levels
- 7×7 — Easy: Compact grid with generous clues. Great for learning the rules and basic Kuromasu techniques.
- 7×7 — Medium/Hard: Same grid size but fewer clue cells, requiring longer deduction chains and cross-referencing.
- 10×10: The classic Kuromasu experience. Balanced clue density with room for complex visibility and connectivity reasoning.
- 14×14: Large grids for experienced solvers. Longer solve times and more intricate interactions between distant clues.
Kuromasu vs Other Logic Puzzles
- vs Nurikabe: Both involve shading cells on a grid with connectivity constraints, but Nurikabe uses island-size clues while Kuromasu uses line-of-sight visibility counts.
- vs Hitori: Both shade cells with an adjacency/connectivity rule, but Hitori eliminates duplicate numbers while Kuromasu uses visibility clues.
- vs Light Up (Akari): Both have line-of-sight mechanics — Light Up places light bulbs that illuminate along rows and columns, while Kuromasu places black cells that block visibility from numbered clues.
- vs Minesweeper: Both use numbered clues that constrain neighbouring cells, but Minesweeper hides information while Kuromasu is fully visible from the start.
History of Kuromasu
Kuromasu was first published by Nikoli, the renowned Japanese puzzle company responsible for popularising Sudoku, Kakuro, Slitherlink, and many other logic puzzles. The puzzle appeared in Nikoli’s flagship magazine Puzzle Communication Nikoli and quickly gained a following among logic puzzle fans. The name “Kuromasu” is a portmanteau of kuro (black) and masu (cells/squares), directly describing the puzzle’s objective.
While not as widely known as Sudoku, Kuromasu has a dedicated community and appears regularly in puzzle competitions. Its combination of visibility counting, adjacency constraints, and connectivity requirements makes it a unique and rewarding challenge among Nikoli’s extensive puzzle catalogue.
Frequently Asked Questions
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