Sudoku techniques · Expert
Hidden Quad in Sudoku
A Hidden Quad is four numbers whose candidates across a unit are confined to the same four cells. Those cells are reserved, so every other candidate in them is removed: the same logic as a Hidden Pair, one size larger.
Why it works
Four numbers whose candidates together appear in only four cells of a row, column, or box claim those cells for themselves.
How to spot it
Work from the counts again, and accept that this one is genuinely rare.
- Four numbers whose combined cell lists touch exactly four cells. Each number needs at least two of them.
- In practice the app finds these far more often than a human will, which is fine: the point of the lesson is to recognise one when a hint shows it to you.
- If a unit has only five blanks, a Hidden Quad is the same statement as a Naked Single in the fifth.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
PatternAnswerContextBlockerElimination
Step 1 of 2
- 1
In this row, 1, 2, 3, 4 appear as candidates only in these four cells.
- 2
These four cells must contain 1, 2, 3, 4 among them, so we can remove any other candidates from those cells.