How to Master Unique Rectangle Type 1 in Sudoku
Advanced Sudoku puzzles are designed with a fundamental rule in mind: a proper Sudoku puzzle must have only one unique solution. If a configuration allows two different solutions, the puzzle becomes invalid. The Unique Rectangle Type 1 technique uses this exact rule to eliminate useless candidates and prevent the grid from falling into an unresolvable state.
The following four Unique Rectangle techniques are quite similar, with only a few small differences. Let’s take a look at the first one – Unique Rectangle Type 1.
What is a “Deadly Pattern”?
A Deadly Pattern occurs when four cells form a rectangle across two 3×3 blocks, two rows, and two columns, where all four cells contain the exact same two candidates.
If a grid reaches this state, the two numbers could be swapped interchangeably without affecting the rest of the board. This creates two equally valid solutions, which breaks the fundamental rule of Sudoku.
Therefore, we must prevent a Deadly Pattern from forming at all costs.
Identifying Unique Rectangle Type 1
To spot Type 1 on the board, look for four cells forming a rectangle across two 3×3 blocks:
Take a look at this example:
- Three Pure Bi-value Cells: Three corners of the rectangle contain only the target pair of digits (
3 and 9). - One Cell with Extra Candidates: The fourth corner contains the same target pair plus one additional candidate).
So we are looking at Unique Rectangle Type 1.
The Elimination Rule
To save the puzzle from entering an unresolvable loop, the fourth cell must solve to one of its extra candidates, automatically rejecting the deadly pair.
If 3 or 9 were placed in this cell, it would create what some refer to as a “deadly pattern” because the puzzle would no longer be unique – we would get more than one valid solution for this grid configuration.
📍 The Action to Take:
So we can safely remove this pair from the cell.
Simply erase the pair 3 and 9 from the pencil notes of that fourth cell. If that cell was left with (3, 9 and 5), removing 3 and 9 instantly resolves that cell into a Naked Single, unlocking the rest of the grid!
Why Unique Rectangles are a Game-Changer
Unique Rectangle techniques allow you to make bold deductions without building long, complicated logical chains. By recognizing the geometry of a Deadly Pattern, you can clear candidates in seconds and keep your solving momentum going.
Ready to break the deadly pattern? Put your logic to the test right now on our Master Sudoku Puzzles, turn on full pencil notes, and hunt for those four-corner traps!