Unique Rectangle
Prerequisites: Naked Pair
Overview
A Unique Rectangle (Type 1) relies on the puzzle having exactly one solution. Four cells in a rectangle across two boxes, none of them givens, can't all end up holding only the same two digits, or they could be swapped for a second solution. So when three corners hold just that pair, the fourth corner can't be either digit.
How It Works
Find four cells forming a rectangle (2 rows × 2 columns) that spans exactly two boxes. If three corners have only the same two candidates, A and B, the fourth corner can't be A or B: if it were, all four corners would hold only A and B, and swapping them would give a second solution. So A and B can be removed from the fourth corner.
When To Use It
Look for Unique Rectangles when you see three bivalue cells with the same candidate pair forming three corners of a rectangle across two boxes. Check the fourth corner for extra candidates.
How To Detect It
Find pairs of bivalue cells with identical candidates. Check if three such cells form three corners of a rectangle spanning exactly two boxes. If the fourth corner contains the same pair plus extras, eliminate the pair digits from it.
Visual Example
Step 1 of 9
Puzzles Have One Solution
A proper sudoku has exactly one solution, and every puzzle in this app does. A Unique Rectangle uses that fact: it rules out a pattern that would give the puzzle a second solution.
Candidates already ruled out by earlier techniques aren't shown.
Practice
Your goal: fill the highlighted cell. Select a cell, then pick a number.
A Unique Rectangle is hiding here. Use the uniqueness rule to eliminate, then fill the Naked Single.Candidates already ruled out by earlier techniques aren't shown.
Use it in a real game
Hard puzzles are where you'll need techniques like this one.
Play a Hard puzzle