Rules

Shikaku Rules Explained

Every valid Shikaku solution follows four rules. First, each region must be a rectangle. Second, each rectangle must contain exactly one numbered clue cell. Third, the area of that rectangle must equal the clue value. Fourth, the rectangles together must cover the whole board with no overlap and no gaps.

A clue of 4 means its rectangle must cover four cells in total. That could be a `1x4`, `2x2`, or `4x1` shape, but it still has to stay on the board and still has to contain only that one clue. This is why some apparent options disappear quickly once you look at neighboring clues.

The “exactly one clue” rule matters just as much as the area rule. A rectangle with the right size is still illegal if it contains two numbered cells. A rectangle with one clue is still illegal if its area is wrong. You always need both conditions at once.

The final full-board coverage rule is where local and global logic meet. You can place a rectangle that looks legal in isolation and still create a dead end for the rest of the puzzle. Strong solving comes from respecting both the local rules and the board-wide consequences.