Hidden Quad

Rarely needed8 min

Overview

Four digits that, in one house, fit only in the same four cells. Those cells must hold those four digits, so every other candidate can be removed from them. Hidden Quads are rare: this lesson is in the Rarely needed tier.

How It Works

If four digits appear only in four cells within a house, those digits must go in those cells. Eliminate all other candidates from the four cells.

When To Use It

Rarely: most puzzles never need one. The house's other empty cells always form a naked set (here a Naked Quad), and that is usually easier to spot.

How To Detect It

For each house, check all combinations of four digits. If the union of their positions equals exactly four cells, it is a Hidden Quad.

Visual Example

Step 1 of 8

Look Inside Box 5

A Hidden Quad is four digits that, in one house, fit only in the same four cells. The eight empty cells of Box 5 (outlined) still need 1, 2, 3, 5, 6, 7, 8 and 9. Count where each one can go.

Where to look

Pattern cells

Eliminations

Removed

7Solved

Key candidates

This step's candidates

Candidates already ruled out by earlier techniques aren't shown.

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Use it in a real game

You'll rarely need this one, but Hard puzzles are where it can turn up.

Play a Hard puzzle