If only one number has any rectangle that covers an empty cell, that cell must belong to that number. Claiming the cell then removes every option of that number that misses it — often leaving just one.
Flip the question
Most beginners only look at numbers. Strong solvers also look at empty cells, especially in corners and pockets. Every cell must be covered by somebody — so a cell with only one candidate is a free deduction.
How to check a cell
- Pick an empty cell, ideally in a corner or a gap between placed rectangles.
- For each nearby number, ask: is there any legal rectangle of this number that covers the cell?
- If exactly one number says yes, the cell is that number’s. If none do, an earlier rectangle is wrong.
A real example
Why it chains
Claiming a cell is not the end. The number now has to include that cell, which usually shrinks its option list to one — a forced rectangle. In our worked example, reachability steps are often followed immediately by forced ones.
A cell that no number can reach is a red flag: one of your rectangles is wrong. Use Check in the game or undo back to your last certain move.
Practise reachability
Play the 6×6 grid from the example.
Questions
What is reachability in Shikaku?
Reachability asks which numbers could possibly cover a given empty cell. A cell with only one possible number belongs to it.
What if no number can reach a cell?
Then the current position is impossible — at least one rectangle drawn so far must be wrong.