Hidden Quad
Four digits whose only possible cells inside a unit are the same four cells. Those four cells must hold those four digits — clear everything else from them.
Intermediate sudoku technique
What it is
A hidden quad is the four-digit analog of hidden triple. Inside a single unit, four digits have candidates restricted to only four cells — the same four. Each of those digits must appear in one of those four cells, so collectively the four cells hold exactly those four digits. The quad is 'hidden' because each of the four cells may still list additional candidates that get masked by the four locked-in digits. The technique's value is eliminating those extra candidates. Hidden quads are rare in practice — most expert-tier puzzles never need them — but when one appears, it unlocks several follow-up moves at once.
When to use it
On hard, expert, and master puzzles after hidden triples are exhausted. The pattern is rare but powerful when found.
Worked example
In column 5, four digits — 2, 5, 6, 8 — each have candidates restricted to only cells R1C5, R3C5, R6C5, R8C5 (each cell may also list 3, 7, or 9 as candidates). The four digits must occupy those four cells in some order. Therefore 3, 7, 9 can be eliminated from R1C5, R3C5, R6C5, R8C5.
Try it
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