---
title: "Unique Rectangle: Avoid a second solution"
description: "Unique Rectangle requires a guaranteed unique solution. This example uses four cells in two boxes to show how that condition rules out a candidate."
lang: "en"
canonical: "https://unisu-sudoku.app/en/sudoku-techniques/unique-rectangle/"
---

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Advanced techniques

# Unique Rectangle: Avoid a second solution

This form of Unique Rectangle requires a puzzle with a guaranteed unique solution. Four cells occupy two rows, two columns and exactly two boxes; three cells contain only the same pair of candidates.

![Unisu lesson on Unique Rectangle, showing the explained grid](https://unisu-sudoku.app/_astro/en.9iFwXbBG_2eRtJS.webp)

This form of Unique Rectangle requires a puzzle with a guaranteed unique solution. Four cells occupy two rows, two columns and exactly two boxes; three cells contain only the same pair of candidates.

## An example from the app

The four corners D4, H4, D5, H5 lie in two rows, two columns and exactly two boxes. Three corners have only 4, 9. Corner D5 has extra candidates.

If D5 also used one of 4, 9, swapping the two digits in the rectangle would produce a second solution. The uniquely solvable puzzle does not allow this. D5 must use another digit.

These notes are therefore ruled out: D5: 4, 9.

The image shows the candidates relevant to this step. A note being hidden does not mean it has been logically eliminated.

## Practice the next step

The fourth cell cannot be left with just that pair, which would allow two interchangeable completions. This Type 1 deduction is not valid without the uniqueness assumption.

In the app, follow the reasoning in the matching lesson and try a task yourself. With a new puzzle, always start from its actual candidate state.

## Questions about Unique Rectangle

### Why does Unique Rectangle need a unique solution?

The elimination prevents a second solution obtained by swapping the pair. Without guaranteed uniqueness, that argument is invalid.

### Why must the corners occupy exactly two boxes?

The pair can then be swapped at the four corners without changing the box constraints. An arbitrary rectangle need not allow this.

### What remains in the fourth cell?

In the illustrated Type 1, remove both pair digits there. If several additional candidates remain, the cell is not yet determined.

### Can I remove the pair from all four corners?

No. This case eliminates from the corner with extra candidates, not from the three pure pair cells.

### What if the other three corners have extra candidates?

The conditions of this Type 1 case are absent. The rectangle’s shape alone proves no elimination.

## Keep practicing in Unisu

Download Unisu for iPhone or Android. Solve a puzzle or work through a guided lesson on the technique.

-   [Download on the App Store](https://apps.apple.com/us/app/id6775824061?l=en)
-   [Get it on Google Play](https://play.google.com/store/apps/details?id=app.unisu.sudoku&hl=en)

## Useful before this guide

### Naked Pair

[Read more: Naked Pair](https://unisu-sudoku.app/en/sudoku-techniques/naked-pair/)

## Where to go next

Advanced techniques

Unisu · Oct 27, 2026

### [Finned X-Wing: An X-Wing with an extra possibility](https://unisu-sudoku.app/en/sudoku-techniques/finned-x-wing/)

A Finned X-Wing adds a fin to the basic pattern. Remove only candidates that conflict with both the fin and the X-Wing left when the fin is false.

Unisu

### [Learn Sudoku: a path from rules to techniques – Unisu](https://unisu-sudoku.app/en/learn/)

Start with the rules, get comfortable with pencil marks and choose a technique for the situation you’re working on.

Download app

## Ready for a puzzle? Take a moment.

Download Unisu on your phone. Start a Sudoku or choose a lesson on the technique you want to understand.

-   [Download on the App Store](https://apps.apple.com/us/app/id6775824061?l=en)
-   [Get it on Google Play](https://play.google.com/store/apps/details?id=app.unisu.sudoku&hl=en)

![The daily challenge in Unisu](https://unisu-sudoku.app/_astro/en.C-sYR9hJ_1QwI9G.webp)

![A classic Sudoku game in Unisu](https://unisu-sudoku.app/_astro/en.C6DJ1kiE_2dRWDN.webp)