Hidden Quad

Overview

Tier 3

Subsets

Typical difficulty

Peak Score 55–70

Also known as

Concealed Quad

When four digits appear only within four cells of a house, those cells are locked for those four digits. All other candidates in those cells are eliminated.

Hidden Quads are the hidden counterpart to Naked Quads. They occur when four digits are restricted to four cells within a house. Because finding a Hidden Quad in a house with six empty cells is the exact mirror of finding a Naked Pair in the other two cells, skilled solvers often look for the smaller pair instead.

How to apply it

Spotting the pattern and making the deduction, in order.

1. 01Identify four digits that appear only within four specific cells of a row, column, or box.
2. 02Ensure that none of those four digits appear in any other cell of that unit.
3. 03Those four numbers must fill those four designated cells.
4. 04Erase all other candidate notes from those four cells.

Common mistakes

• !Spending too much time hunting for hidden quads when checking the remaining cells for a naked pair is much quicker.
• !Failing to verify that none of the four digits leak into a fifth cell.

Frequently asked questions

Is it better to look for Hidden Quads or Naked Pairs?

In a house with 6 empty cells, a Hidden Quad and a Naked Pair are two sides of the same coin. Looking for the 2-cell Naked Pair is almost always faster and less error-prone.

Related techniques