Sudoku technique guide

Hidden Quad Sudoku Technique

A hidden quad is four candidates that only appear in the same four cells within one area. 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 Hidden Quad normally sits in a human solving path.

Family

Subset techniques

It belongs to the subset 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 Hidden Quad?

A hidden quad is four candidates that only appear in the same four cells within one area.

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 Hidden Quad works

Find four candidates in one area.
If they only appear in the same four cells, those cells are reserved.
Remove all other candidates from those four cells.

When to look for it

Look for Hidden Quad 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

Hidden quads are rare and messy. Check that the four candidates do not appear elsewhere in the area.

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.

Hidden Quad FAQ

What is Hidden Quad in Sudoku?