What is Unique Rectangle? How to Solve Sudoku

What it is

Uses the rule that a puzzle has exactly one solution.

If the four corners of a rectangle (across two boxes) could each only be 3 or 7, you could swap the 3s and 7s and still be correct — two solutions.

To avoid that, you can rule some candidates out.

Example: three corners can only be 3 or 7, and the fourth can be 3, 7, or 9 → the fourth corner is 9.

How to spot it

Look for cells with the same two candidates (like 3 and 7) sitting on three or four corners of a rectangle.

Example

Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.

R3C1, R3C3, R8C1, and R8C3 are the corners of a rectangle. If all four could only be 3 or 6, you could swap them and still be right — two solutions. A puzzle has only one, so R3C3 is neither 3 nor 6. Remove 3 and 6 from R3C3.