Naked Quad
Prerequisites: Naked Triple
Overview
Four cells in a house whose combined candidates contain at most four digits. Those digits can be eliminated from all other cells in the house.
How It Works
Identical logic to naked pair and triple, extended to four cells. If four cells collectively contain only four distinct candidates, those digits are locked to those cells. As with the triple, each cell need not contain all four digits -- any mix works as long as their union is exactly four, e.g. (W,X), (X,Y), (Y,Z), (W,Z).
When To Use It
When simpler subsets have been exhausted and four cells in a house share a limited set of candidates.
How To Detect It
Check combinations of four cells in a house. If their candidate union contains exactly 4 digits, eliminate those digits from remaining cells.
Visual Example
Step 1 of 8
Extending to Four Cells
A Naked Quad follows the same logic as a Naked Pair and Naked Triple, extended to four cells. If four cells in a house collectively contain at most four distinct candidates, those digits are locked to those four cells.