Sudoku technique guide

Empty Rectangle Sudoku Technique

An Empty Rectangle combines a box shape with a strong link to remove one candidate. This guide explains the idea in plain English, when to look for it, and the common mistake to avoid.

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At a glance

Difficulty

Hard

This is where Empty Rectangle normally sits in a human solving path.

Family

Single-digit chain techniques

It belongs to the single-digit chain techniques group of Sudoku logic.

Main action

Usually remove candidates

Most technique moves reduce notes first. That often reveals a simpler placement afterwards.

What is Empty Rectangle?

An Empty Rectangle combines a box shape with a strong link to remove one candidate.

The important thing is that this is logic, not guessing. You are using the current candidates to prove that a number must either go in one place, or cannot stay in another place.

In SudoSketch, this technique is designed to work with notes and Coach highlighting, so the key cells and removal cells can be shown clearly.

How Empty Rectangle works

Focus on one candidate in a 3x3 box.
The candidate positions form a row-and-column shape with one missing corner.
Combine that box shape with a strong link to remove one outside candidate.

When to look for it

Look for Empty Rectangle after easier moves have stopped working. First check for naked singles, hidden singles, locked candidates and simple pairs. If those do not move the puzzle forward, this technique may be worth checking.

For learning, do not try to scan for every advanced technique at once. Pick one method, understand the shape, and practise spotting that one pattern.

Common mistake

The box pattern and the outside strong link both matter. Do not use the box shape alone.

If you are unsure, rebuild the candidates first. Bad notes create bad logic, and Sudoku will absolutely punish you for it like a tiny spreadsheet goblin.

Empty Rectangle FAQ

What is Empty Rectangle in Sudoku?