In short. Shikaku is played on a grid where a few squares carry a number. You cut the whole grid into rectangles so that each rectangle holds exactly one number, and its area in squares equals that number. No rectangle may overlap another, and no square may be left out. A correct grid has exactly one solution, and that solution is reached by deduction: guessing is never required.
- 1Number per rectangle. Not two, not zero.
- 4Deductions, and the whole grid goes down.
- 0Coin flips. Every grid is checked before it ships.
The rule fits in three lines
You cut the whole grid into rectangles. Each rectangle holds exactly one number. And that number is its area: its count of squares. There, the training is over.
So a 6 is a rectangle of six squares: 1 × 6, 2 × 3, 3 × 2 or 6 × 1. A square counts as a rectangle, yes, and so does a strip one square wide. Your school geometry has your back.
Nothing overlaps, nothing is left over. When the last square finds a taker, that's it. And there was only one way to get there, which is still the best news of the day.
The four deductions that are enough
A Shikaku is not solved by picturing the whole partition. That method is reserved for people who can see the Matrix. It is solved by elimination, number by number and square by square. Four lines of reasoning are enough, and they are exactly the four the game's solver applies to check a grid before sending it your way.
1. The only rectangle that fits
Take a number and count the rectangles of the right area that contain it without swallowing another number or leaving the board. If one is left, it is placed, and you didn't even have to think. This is the big moment for prime numbers against an edge: a 5 has only two shapes, 1 × 5 and 5 × 1, and the edge often kills one.
2. The square with a single suitor
Switch the point of view: look at an empty square and ask which rectangles, across all numbers, can still cover it. If only one is left, it is placed, whatever room it takes elsewhere.
3. The square only one number cares about
The most rewarding of the four, and the one nobody practises. The square may have several suitors, but if they all belong to the same number, then that number is going to take it. And just like that, you throw away every one of its rectangles that misses the square.
. . . . . 6 . . . . . 3 ? . . .
The 3 cannot get down to the corner, its three-square rectangles stay too high. The 6 can, with a 2 × 3 along the left edge. The corner therefore belongs to the 6, and every layout of the 6 that avoids it goes straight in the bin. One square examined, a pile of possibilities gone.
4. The forced crossing
The reverse move. Take a number and stack in your head all the rectangles it can still take. If a square turns out to be covered in every case, it already belongs to that number, even if the final shape is still a mystery. Nobody else gets to claim it.
Where to start
- Corners and edges first. A corner square can be reached by very few rectangles, often from a single number. That is where the third deduction pays best.
- Then the primes. 2, 3, 5, 7, 11: two orientations, not one more, and an edge regularly kills one.
- Big numbers after that. A 12 has many shapes but it needs room: on a crowded grid it often finds only one, and it stops showing off.
- Sweep the lonely squares again. After each piece, look at the ones that just lost suitors. That is where the grid starts unfolding on its own, and it feels very good.
The mistake that costs the game: placing a rectangle because it fits, without checking that it is the only one that fits. The game accepts it, of course, the area is right. And the error hits you in the face ten pieces later, when a square has no taker left. Finding which of the ten is wrong then costs far more than the deduction you saved. Don't ask me how I know.
Never ask "where can this number go?". Ask "who can cover this square?". Same grid, same rules, but the second question has far fewer answers. That is the whole job.
Why you never guess here
A correct Shikaku has a unique solution and, more importantly, a path to it: at every step at least one deduction is available. Without that guarantee the game becomes a bet, and a bet lost after twenty minutes stays with you.
Here every level is generated and then put through the solver before it ships. If the slightest assumption is needed to move forward, the grid goes in the bin and another one is generated. Uniqueness comes along for the ride: when every step is forced, there never was a branch.
Where Shikaku comes from
The puzzle was invented in 1989 by Yoshinao Anpuku, then a maths student at Kyoto University, and published by Nikoli, the Japanese publisher we owe the sudoku craze to. Its full name, 四角に切れ, shikaku ni kire, means "cut into rectangles". At Nikoli, puzzles are named the way you write a user manual.
You will meet it elsewhere as "Rectangles" and "Divide by Box", sometimes "Cell Block". Same game, same rules, just a different outfit.
In this game
The gesture
Finger on a square, drag, the rectangle draws itself. A badge shows the running area against the target, 4 / 6, and it sits above your finger, never below. Because on a phone your hand covers exactly what you are trying to read, and I got caught out before you did. Let go when the count is right. To lift a piece already set, tap inside it.
What the game refuses, and why it tells you
Wrong-sized rectangle? Refused, with the reason: "the 6 needs 6 squares, you drew 8". Two numbers inside, it says that too. This is deliberate: a silent refusal reads as a bug and you go complain on the store. A refusal that explains itself teaches the rule in one session.
The hint explains before it answers
The first press sets nothing at all. It marks a square and hands you the deduction in plain words: "only the 6 can reach this square, so its rectangle has to cover it". The second press places the piece, if you really do want the answer. And no, hints cost no stars. That would be a bit mean.
The stars
Three stars means a clean run. These count as mistakes: a rectangle refused for the wrong area or for swallowing two numbers, and above all a piece set and then lifted, which is admitting it was wrong. A finger sliding off the board does not. We punish the reasoning, not the clumsiness.
The silhouettes
Some grids are not rectangles but cut-out shapes: a heart, a key, a cat. They are not harder, they are actually easier, because every hollow edge removes possible rectangles, so there is less to examine. They are the campaign's breathing room, not its peaks.
Frequently asked questions
What is Shikaku?
A Japanese logic puzzle published by Nikoli. You cut a grid into rectangles so that each rectangle holds a single number equal to its count of squares. Nothing overlaps and no square is left out.
Does a square count as a rectangle?
Yes. A 4 can be a 2 × 2 square, a 1 × 4 or a 4 × 1 strip, and a 9 can be a 3 × 3 square. The word rectangle includes the square, as in geometry.
Do you ever have to guess?
No, never, if the grid is correct. Every level in this game goes past a solver that only applies forced deductions: if anything has to be assumed to move on, the grid is thrown away before it is served.
How do I know where to start?
Corners and edges, then prime numbers. A corner square can be reached by very few rectangles, and a prime has only two possible orientations, one of which the edge often removes.
Does Shikaku go by other names?
Yes: "Rectangles", "Divide by Box", sometimes "Cell Block". Its full Japanese name is 四角に切れ (shikaku ni kire), "cut into rectangles".
How is it different from a nonogram?
Both are solved by pure deduction, but a nonogram has you fill squares from runs of numbers in the margins and reveals a picture. Shikaku cuts the surface into regions: you are not looking for a drawing, you are looking for a partition.
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