Quick facts
| Japanese name | 四角に切れ (Shikaku ni Kire) |
|---|---|
| Other names | Rectangles, Divide by Squares, Divide by Box, Sikaku |
| Inventor | Yoshinao Anpuku (sources also spell it “Yoshiano”) |
| First published | 1989, Puzzle Communication Nikoli |
| Publisher | Nikoli Co., Ltd., Tokyo |
| Genre | Region-division logic puzzle |
| Computational complexity | NP-complete (for arbitrary grid sizes) |
Origins at Nikoli
Shikaku was devised by Yoshinao Anpuku, then a mathematics student at Kyoto University, and published by the Japanese puzzle company Nikoli in 1989. According to a Nikoli staff member, it first appeared in Puzzle Communication Nikoli Vol. 27, issued in September 1989. The idea is said to draw on an older Japanese figure puzzle known as Tachiawase.
Nikoli is the publisher that popularised Sudoku in Japan and created or refined many of the best-known “pencil puzzles”, including Kakuro, Slitherlink, Nurikabe, Hashiwokakero and Masyu. Like those puzzles, Shikaku follows the Nikoli philosophy: simple rules, a unique solution, and a solving path that relies on logic rather than trial and error.
What the name means
Shikaku (四角) means “square” or literally “four corners” in Japanese. The full title Shikaku ni Kire (四角に切れ) is usually translated as “divide by squares” or “cut into rectangles”, which describes the goal exactly. English-language publishers have used names such as Rectangles, Divide by Squares and Divide by Box for the same rules.
Going global
After the Sudoku boom of the mid-2000s, newspapers, puzzle magazines and apps around the world went looking for the “next Sudoku”, and many Nikoli genres — Shikaku among them — found a large international audience. Today Shikaku is a staple of logic-puzzle books, puzzle apps and competitive puzzle events, and it is often recommended as a gentle introduction to region-division puzzles.
The mathematics of Shikaku
Despite its friendly rules, deciding whether an arbitrary Shikaku grid has a solution is NP-complete: as grids grow, no known algorithm can solve every instance efficiently. In practice, published puzzles are designed to be solvable by step-by-step deduction, which is why humans can enjoy even large grids.
Puzzle constructors also care about uniqueness. A good Shikaku has exactly one solution. On PuzzleShikaku we guarantee this by running every puzzle through an exhaustive solver; you can read more on our methodology page or try the online Shikaku solver yourself.
Related puzzles
| Puzzle | Similarity to Shikaku |
|---|---|
| Fillomino | Divide the grid into regions whose size matches a number — but the regions can be any shape. |
| Tatamibari | Another Nikoli rectangle puzzle, where symbols say whether each rectangle is a square, wide or tall. |
| Nurikabe | Numbers give the size of “islands” of white cells separated by a connected sea. |
| Sudoku | Shares the unique-solution, logic-only philosophy, but with digits instead of regions. |