Shikaku Puzzle divide the grid by area
Shikaku is a clean spatial deduction puzzle: divide every cell of the board into rectangles so each rectangle contains exactly one number and has exactly that many cells. The rule set is small, but the interaction between possible widths, heights, clue positions and neighboring rectangles creates a surprisingly deep deduction loop.
Play the full Shikaku specialist
The main game includes Easy 6×6, Standard 8×8 and Challenge 10×10 boards, seeded Practice URLs, drag-to-draw controls, keyboard rectangle selection, Undo/Redo, Check, a technique-aware Verified Hint, local autosave and a live candidate monitor. The source bank is generated inside the project and accepted only when an independent exact-cover solver counts exactly one complete partition.
Shikaku rules in plain English
Every numbered cell belongs to one rectangle. The number tells you the rectangle’s area. A clue of 6 can therefore belong to a 1×6, 2×3, 3×2 or 6×1 rectangle if that shape fits on the board. A clue of 5 can only use 1×5 or 5×1. A clue of 12 may have many more possibilities. Each chosen rectangle must contain exactly one clue, rectangles cannot overlap, and the finished set of rectangles must cover every cell with no gaps.
The clue can sit anywhere inside its rectangle. It does not need to be centered and does not identify a corner. That is why edge and corner clues are often informative: the board boundary removes many otherwise possible placements. Nearby clues also remove possibilities because a legal rectangle cannot swallow a second numbered cell.
Start with factor pairs
The fastest first question is not “where does this rectangle go?” but “what shapes can this number have?” Factor the clue into height-by-width pairs. For 8, the basic shapes are 1×8, 2×4, 4×2 and 8×1. On a 6×6 board the 1×8 shapes are impossible immediately. If another clue blocks part of a 2×4 placement, that candidate disappears too. Shikaku rewards this kind of shrinking-candidate reasoning.
The V0.124 live monitor performs the same geometry without revealing the stored solution. For each open clue it enumerates rectangles with the correct area, requires exactly one clue inside, then removes candidates that collide with rectangles you have already committed. The displayed candidate count is therefore derived from the visible puzzle state rather than from a hidden answer key.
Look for forced rectangles
A clue with exactly one remaining candidate is forced. Draw it immediately, then recalculate the pressure on neighboring clues. One rectangle can remove several candidate shapes around it, causing a chain of new forced placements. This is the most satisfying Shikaku rhythm: factor possibilities, place a forced region, then watch the geometry tighten elsewhere.
Small prime clues are often useful because they have few shape families. Board-edge clues can also be strong because many placements extend outside the grid. Large clues may look difficult, but a large rectangle can become forced when surrounding clues block almost every possible location.
Use uncovered cells as evidence
Do not reason only from clue cells. An ordinary empty cell eventually has to belong to one rectangle. If every candidate rectangle covering a particular open cell belongs to the same clue, that clue must eventually absorb the cell even if the final boundaries are not yet obvious. Likewise, if a committed rectangle creates an area that no remaining clue can possibly cover, you have learned that one of your earlier choices is wrong.
The game’s Check button can compare committed rectangles with the verified solution, but using Check marks the run Assisted. For clean solving, use the candidate counts and the geometry first. Undo and Redo are available so you can explore a hypothesis without losing your previous state.
How Verified Hint works
Verified Hint first searches the current state for an open clue with exactly one remaining legal rectangle. When it finds one, the hint explains that the placement is forced and draws that rectangle. This is a technique-aware hint because the reason can be reproduced from the same candidates shown in the monitor.
If no clue is immediately forced, the hint chooses an open clue with a low remaining-candidate count and supplies one solver-verified placement. That second kind of hint intentionally marks the run Assisted; it is useful when you want to keep learning instead of revealing the entire board.
Why unique-solution verification matters
A Shikaku board can look reasonable while still allowing two different complete partitions. A user should not be blamed for choosing a valid alternative when the puzzle author failed to prove uniqueness. V0.124 avoids that problem by separating generation from verification. The project-local generator creates a rectangular tiling and clue positions, while an independent exact-cover solver independently enumerates legal clue rectangles and counts complete non-overlapping covers.
Only boards with exactly one full cover are frozen into the source bank. The release verifier then checks all 60 sources, all eight symmetry transforms of every source, thousands of deterministic Practice selections and the full 2026 Daily schedule. This does not make every puzzle equally difficult, but it makes the core solvability claim testable.
Practice modes and controls
Easy uses 6×6 boards, Standard uses 8×8 and Challenge uses 10×10. Practice URLs stabilize with a seed so the same URL recreates the same source and symmetry transform. On pointer devices, drag from one corner of a rectangle to the opposite corner. On keyboard, move the cursor with the arrow keys, press Enter to set the first corner, move to the opposite corner and press Enter again. Escape cancels a pending rectangle and Delete removes the rectangle under the cursor.
Progress is saved locally in the browser. There is no account requirement and the game does not need a paid puzzle API or remote solver at runtime. The specialist Daily is separate from the platform’s frozen Today’s 6 rotation, so adding Shikaku does not silently change previously scheduled Daily lineups.
What makes a good Shikaku solve?
A fast time can be fun, but the stronger goal is a traceable chain of deductions. Identify the most constrained clue, place a rectangle only when its geometry is justified, then use the newly occupied space to reduce candidates elsewhere. On harder boards, alternate between clue-first reasoning and cell-coverage reasoning instead of staring at one number for too long.
If you are new, start with Easy and leave the candidate monitor visible. Once you can anticipate why a clue will drop from three candidates to one, move to Standard. Challenge boards are larger and intentionally contain more ambiguous clues, so they reward maintaining several possibilities before a later rectangle resolves them.