The Hidden Quad Technique identifies four candidates that are spread across exactly four cells in the same row, column, or block. These four candidates, in some combination, are the only places where these candidates can appear within that house, meaning no other candidates are necessary in those four cells.
- Row-based Hidden Quad: If four candidates appear only across four specific cells of a row and nowhere else in that row, all other candidates in those four cells can be eliminated.
- Column-based Hidden Quad: If four candidates are distributed across four cells in a column and no other cells in the column contain those candidates, the remaining candidates in the four cells can be eliminated.
- Block-based Hidden Quad: If four candidates are found across four cells in a block, these four candidates must be confined to those cells, allowing you to eliminate other candidates from them.
In this puzzle, a Row-based Hidden Quad occurs where the numbers 1, 4, 7 and 8 appear in exactly 4 columns in row 3. As a result, these numbers must occupy those cells, and all other candidates can be eliminated from those cells.
This puzzle combines Hidden and Naked Singles with one Hidden Quad technique for a challenging and rewarding solve.
How to spot it
Find four digits in a unit that are confined to the same four cells, regardless of whatever other candidates those cells display.
When to use it
It is the rarest of the hidden subsets; look for it in dense units where four digits cluster into four cells.
Common mistakes
- Confusing it with a Naked Quad; the Hidden Quad's cells carry extra candidates that you must remove.
- Failing to confirm the four digits appear nowhere else in the unit.
Frequently asked questions
- What is a Hidden Quad in Sudoku?
- A Hidden Quad is four digits that can only be placed within the same four cells of a unit. All other candidates are then removed from those four cells, leaving just the quad.
- Why are Hidden Quads so hard to see?
- The four cells usually contain many extra candidates that mask the pattern, so you must track where each digit can go across the whole unit to find them.
- How is a Hidden Quad different from a Naked Quad?
- A Naked Quad shows only the four digits in its cells. A Hidden Quad hides the four digits among extra candidates; applying it clears those extras.