Hidden Quad Walkthrough
A Hidden Quad is four digits that appear only within the same four cells of a unit. Those cells are reserved for those digits, so other candidates can be cleared from them.
There is a Hidden Quad in Row 3. Applying the Hidden Quad eliminates 4 candidates (struck out in red below; any solved cells shown in green).
Reading this board
The pattern on this puzzle involves row 3. Work from the before-image above: find the hidden quad for yourself before scrolling to the after-image, since the point of a worked example is the search, not the answer. No cell is solved outright by this move. The value is positional: 4 impossible candidates leave the grid, which is what makes the next round of scanning productive.
Why the deduction is valid
Four digits that can only be placed in the same four cells of a unit must occupy them, so every other candidate in those four cells is impossible. It is the largest of the hidden subsets and by far the rarest, because it requires four digits to be simultaneously restricted — a condition that usually resolves through some simpler technique first.
How to spot a Hidden Quad on your own board
- Tally which cells can hold each missing digit in the unit, then look for four digits whose combined footprint is exactly four cells.
- Expect to find it only in units with six or more empty cells, where the simpler subsets have not already cracked things open.
- In practice it is worth checking for a Hidden Quad only when a puzzle is genuinely stuck and every cheaper technique has been ruled out.
Common mistakes
- Reaching for it too early. If a Hidden Pair or an Omission is available, it will be faster and will often dissolve the quad before you need it.
- Forgetting the action is subtractive inside the four cells, not across the unit.
What to do next
The four cells become a Naked Quad once cleaned. Sweep the unit for singles and check whether the cells share another unit where the quad also applies.