Hidden Quad
Intermediate8 min
Prerequisites: Hidden Triple
Overview
Four candidates confined to exactly four cells within a house. All other candidates can be removed from those cells.
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
Extremely rare. Usually the complementary naked subset would be found first.
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
Four Digits Confined to Four Cells
A Hidden Quad occurs when four digits are collectively confined to the same four cells within a house. Each digit appears in two, three, or all four of those cells — but nowhere else in the house.
Pattern cells
Affected cells
Eliminations
Key candidates