Difficulty
Expert
Sudoku technique guide
Hidden Quad
When the easy moves slow down, one clear pattern can make the next step feel possible again. Hidden Quad helps you spot what the grid is already telling you.
By the end, you will know what to notice, how the idea works and when it is worth using.
At a glance
Technique family
Hidden subsets
Purpose
Removes candidates
Typical puzzle stage
Late
Frequency
Rare
Prerequisites
Hidden Triples and disciplined candidate scanning
Why this technique matters
Hidden Quads are an expert-level cleanup tool. The main lesson is restraint: use it when the evidence is already tight, not as a fishing expedition.
The idea in plain English
A Hidden Quad is four digits confined to four cells in one unit. Those cells may contain extra candidates, but the extra candidates can be removed.
How to recognise it
The four cells often look messy. The pattern appears only after you compare the positions of the digits across the whole unit.
Pause when you see this shape. You do not have to force it; just check whether the pattern is complete.
How the logic unfolds
Think of this as a short conversation with the grid. Take one check at a time, and let each answer tell you what to inspect next.
Start by. List missing digits in one unit and note their candidate positions.
Then. Identify four digits whose combined positions occupy exactly four cells.
Next. Verify none of the four digits has another possible location in the unit.
Finally. Strip all non-quad candidates from those four cells.
When it usually appears
Consider it late in a solve, after hidden pairs and triples have been checked.
Common traps
These are easy places to get turned around. They are useful checks, not signs that you are doing badly.
- Treating any four crowded cells as a quad without confirming the four digits are confined to them.
- Removing a candidate from outside the four cells; the elimination is inside the selected cells.
Frequently asked questions
Is a Hidden Quad the same as a Naked Quad?
No. The starting evidence is different: Hidden Quads begin with digit positions, Naked Quads with cell candidate sets.
Should beginners memorise it?
Recognise the idea, but prioritise the more frequent techniques first.
Quick takeaway
- A Hidden Quad belongs to the hidden subsets family.
- Its purpose is to remove candidates.
- Look for a complete pattern, not one that only nearly fits.
- It is most useful late.
- Once you make the deduction, return to the simpler scans.