Hidden Quad
The Hidden Quad is the rarest and trickiest of the subset techniques — the four-digit cousin of the hidden triple. Four digits can go in only the same four cells of a unit, hidden among plenty of other candidates. Find it and four cells clear at once.
What is a Hidden Quad?
If four numbers — say 1, 3, 6 and 8 — can each only appear in the same four cells of a unit, then those four cells must hold those four digits between them. Every other candidate in those four cells can be deleted. It's the hardest subset to see because the cells stay busy right up until you strip them.
The digits 1, 3, 6 and 8 appear only in the four indigo cells. So those cells hold {1,3,6,8} — delete the stray 5 and 9 from them.
How to find it
- Keep full pencil marks — a hidden quad is impossible to see otherwise.
- Track four digits that between them fit in only four cells of a unit.
- Confirm none leaks out to a fifth cell.
- Clear everything else from those four cells.
In practice, a hidden quad is unusual — by the time four digits are this constrained, a simpler technique has often already done the job. It's worth knowing mainly to complete the family, and for the occasional grid that hides one.
Common mistakes
- One of the four appears elsewhere. All four digits must be confined to the four cells — no exceptions.
- Reaching for it too early. Exhaust singles, pairs and triples first; a hidden quad is a last resort among subsets.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.