THE SHORT VERSION
Shikaku: a grid with scattered number clues. Divide the entire grid into rectangular regions — no overlaps, no leftover cells. Each rectangle contains exactly one number, and that number equals the rectangle's area. A "6" must be inside a 1×6, 2×3, 3×2, or 6×1 rectangle.
Hitori: a grid fully pre-filled with numbers (some repeated). Shade cells so that (1) no number appears more than once in any row or column among the unshaded cells, (2) no two shaded cells share an edge, and (3) all unshaded cells form a single connected group.
Shikaku adds structure (rectangles carve territory). Hitori subtracts structure (shading removes cells). Both have exactly one valid solution.
SIDE BY SIDE
Starting state
Shikaku: sparse grid (a few number clues, most cells empty)
Hitori: dense grid (every cell already has a number)
Player action
Shikaku: draw rectangle borders to assign cells to regions
Hitori: shade individual cells to eliminate them
Direction of solving
Shikaku: additive (assign each cell to a rectangle)
Hitori: subtractive (remove cells by shading)
Constraint count
Shikaku: one rule (rectangle area = clue value)
Hitori: three rules (uniqueness + no-touch + connectivity)
Primary technique
Shikaku: factor the clue, eliminate impossible rectangle shapes by position
Hitori: find duplicate pairs, shade one, cascade via no-touch and connectivity
WHAT STAYS THE SAME
Both puzzles are purely spatial. There is no arithmetic to perform, no digit to calculate — the given numbers are constraints, not values to manipulate. In Shikaku, a "12" tells you the rectangle's area; in Hitori, a repeated "4" tells you at least one of those cells must be shaded. You read the numbers as rules, not as quantities.
Both are solved by elimination. In Shikaku, you rule out rectangle orientations that conflict with adjacent clues until only one shape fits each clue's position. In Hitori, you determine which cell in a duplicate pair must be shaded by checking what the other two rules (no-touch, connectivity) would allow. In both cases, logical pressure builds from the most constrained cells outward.
Neither puzzle involves guessing. Every shading in Hitori and every rectangle in Shikaku can be deduced from the rules alone.
HOW THE REASONING DIFFERS
Shikaku reasoning is geometric. Each clue has a small set of valid rectangle shapes for its position. Edge and corner proximity immediately cuts those options. Adjacent clues further restrict each other's available rectangles. The solving loop is: factor the clue → enumerate candidate rectangles → eliminate those that conflict with neighbours → place the one that remains. It is methodical and visual.
Hitori reasoning is a three-rule cascade. The uniqueness rule (no-repeat in a row/column) generates initial shade candidates. The no-touch rule then constrains which candidate can be shaded — if shading a cell would force two shaded cells to touch, that cell is safe (unshaded). The connectivity rule adds a global check — shading a cell that would disconnect the unshaded region is also forbidden. All three rules feed into each other. A deduction from one rule frequently unlocks a deduction in another.
The key practical difference: Shikaku forces are mostly local (a clue's rectangle affects its immediate neighbourhood). Hitori forces can be global — connectivity checks require you to scan the whole grid, not just the area near the current pair.
WHICH IS HARDER?
Hitori is generally harder, because it has three interacting rules where Shikaku has one. The connectivity check is the hardest individual step in either puzzle — requiring you to mentally trace the unshaded cells across the whole grid to verify they stay connected. Shikaku's single area rule is more tractable; once you know the rectangle dimensions, the placement is a constrained geometry problem.
That said, large Shikaku grids with many large-area clues can be deceptively hard, because each large clue has more valid rectangle shapes and the interactions between neighbouring large rectangles multiply. Hard Hitori on a 10×10+ grid with many duplicate pairs and tight connectivity constraints is among the most demanding spatial puzzles in GridJoy's catalogue.
WHEN TO MAKE THE SWITCH
If you play Shikaku: try Hitori when you want constraint cascades instead of geometry. Hitori's three-rule interaction is a different kind of spatial logic — one where the rules reinforce each other rather than following a single clean area rule. If Shikaku's geometry feels predictable, Hitori's cascade keeps you guessing longer.
If you play Hitori: try Shikaku when you want visual territory-carving instead of cell-elimination. The shift from "remove cells" to "assign cells to rectangular regions" is a complementary spatial skill. Shikaku's geometry rewards the same visual pattern-matching that Hitori uses, applied in an additive direction.