Hidden Quad Practice Puzzles

Every puzzle below is verified to feature the Hidden Quad as its hardest required technique — of the 65,284 puzzles in our library, 405 require it and 379 need nothing harder. New to the pattern? Learn the Hidden Quad or see a worked example first.

How to drill the Hidden Quad

The Hidden Quad is the rarest pattern this site indexes: of the whole verified library, only a few hundred puzzles need one as their hardest required move. That scarcity is the reason the page exists — if you want to see one at all, waiting for it to appear in ordinary play is not a plan.

The pattern is four digits confined to the same four cells of a unit, permitting every other candidate in those four cells to be removed. Finding one honestly requires the full counting pass: for each digit still missing from the unit, list the cells that could hold it, then look for four digits sharing the same four-cell footprint.

Treat these as a curiosity and a test rather than a staple. Their real value is that a solver who can find a Hidden Quad has completely internalised the counting-by-digit habit, at which point the far more common hidden patterns stop being difficult at all.

Curated Hidden Quad puzzles

  1. Practice puzzle #1 — 26 givens · needs Naked Single, Hidden Single, Hidden Quad
  2. Practice puzzle #2 — 23 givens · needs Naked Single, Hidden Single, Hidden Quad
  3. Practice puzzle #3 — 26 givens · needs Naked Single, Hidden Single, Hidden Quad
  4. Practice puzzle #4 — 24 givens · needs Naked Single, Hidden Single, Omission, Hidden Quad
  5. Practice puzzle #5 — 23 givens · needs Naked Single, Hidden Single, Omission, Hidden Quad
  6. Practice puzzle #6 — 23 givens · needs Naked Single, Hidden Single, Omission, Hidden Quad
  7. Practice puzzle #7 — 23 givens · needs Naked Single, Hidden Single, Omission, Hidden Quad
  8. Practice puzzle #8 — 24 givens · needs Naked Single, Hidden Single, Omission, Hidden Quad
  9. Practice puzzle #9 — 27 givens · needs Naked Single, Hidden Single, Naked Pair, Hidden Quad
  10. Practice puzzle #10 — 27 givens · needs Naked Single, Hidden Single, Omission, Hidden Quad
  11. Practice puzzle #11 — 23 givens · needs Naked Single, Hidden Single, Omission, Hidden Quad
  12. Practice puzzle #12 — 24 givens · needs Naked Single, Hidden Single, Naked Pair, Hidden Quad
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Hidden Quad practice FAQ

How many Sudoku puzzles require the Hidden Quad technique?
Of the 65,284 puzzles in the Precision Sudoku library, 405 require the Hidden Quad to solve, and 379 can be completed with nothing harder than the Hidden Quad. Every puzzle is verified by a solver that records the exact techniques needed.
How rare is a Hidden Quad in Sudoku?
Very. Only a few hundred puzzles in a library of over sixty thousand require one as their hardest technique, which is why they are almost never encountered by chance. Every puzzle on this page is verified to need one.
Is there a shortcut for spotting Hidden Quads?
Not really, and that is the point of the exercise. It needs a complete pass listing where each missing digit can go in the unit, then a search for four digits sharing a four-cell footprint. If a shortcut worked, the pattern would not be a useful test of the counting habit.