How to Master Unique Rectangle Type 4 in Sudoku

When extra candidates fill two corners of a rectangle, you don’t always need to track those extra numbers. The Unique Rectangle Type 4 technique flips the script: instead of focusing on the extra digits, it analyzes the strong links of the base digits themselves across the unit.

This Unique Rectangle technique takes other candidates within the unit into account.

Identifying Unique Rectangle Type 4

Look for four cells forming a rectangle across two 3×3 blocks, two rows, and two columns:

• Two Pure Corner Cells: Contain strictly the base pair (5 and 6).
• Two Modified Corner Cells: Contain the base pair plus one or more extra candidates.

We look for additional candidates in two Unique Rectangle cells again, but this time we ignore the extra candidates and focus on the UR candidates themselves.

The Logic Behind the Strong Link

To use Type 4, inspect the shared unit (row, column, or 3×3 block) that holds both modified corners. Look at your two base candidates (5 and 6) across that unit:

If one of the UR candidates is no longer possible in any other cell of a unit that contains both cells with the extra candidates, the other UR candidate can be eliminated from those UR cells.

The candidate 6 cannot be placed anywhere else inside the block, so we cannot delete it from our Unique Rectangle cells.

Because 6 is forced to land in one of those two rectangle corners inside the block, 6 is mathematically locked there.

Now, if those two cells also turned out to hold the digit 5, both cells would collapse into the base pair (5 and 6), instantly completing a 4-corner Deadly Pattern and breaking the puzzle’s uniqueness!

The Elimination Rule

To prevent the Deadly Pattern from forming, the second base candidate must be banished from the modified corners.

This means that we need to delete 5.

📍 Steps to Clean the Grid:

1. Find the Locked Base Digit: Identify which of the two base candidates (e.g., 6) appears only within the rectangle’s corners for that shared row, column, or block.
2. Protect the Uniqueness: Recognize that this locked digit forces the removal of its partner base candidate (5) from those same two corners.
3. Erase from Pencil Notes: Delete candidate 5 from the modified corner cells. (Note: any extra non-base candidates in those cells remain untouched!).

Master Advanced Sudoku Logic

Unique Rectangle Type 4 demonstrates how a constraint in the surrounding grid can be used to clear base candidates inside a pattern. Spotting these locked digits lets you dismantle potential deadly loops effortlessly and opens up clear paths to victory.

Ready to test your scanning skills? Put your logic to the work right now on our Web Sudoku puzzles, activate full pencil notes, and look for those locked base candidates!