If every remaining rectangle for a number covers the same cell, that cell belongs to the number — even while its final shape is unknown. Other numbers can no longer use the cell, which often forces them.
The rule
List a number’s remaining options and find their intersection. Any empty cell in the intersection is guaranteed. On our site, Medium puzzles are graded as needing this technique at least once.
A hand-made example
The 4 can’t be a horizontal strip (it would swallow the 2), leaving two squares and a vertical strip. All three share the cell below the 4.
From a real puzzle
When to use it
- A number has two or three options and they point the same way.
- A large number sits in a narrow corridor — most of the corridor is shared.
- Forced moves and reachability have run dry.
Practise overlap
Play the medium 6×6 grid from the example.
Questions
What is the overlap technique in Shikaku?
Finding cells that every remaining option of a number covers. Those cells belong to that number even before its exact rectangle is known.
Is overlap needed for easy Shikaku?
No. Our Easy puzzles are verified to be solvable without it; Medium and Hard puzzles use it.