Hidden Quads
Short answer: a hidden quad is four shapes that, within one row, column or box, fit in only four cells between them. Those four cells must take those four shapes in some order, so anything else pencilled into them is impossible. Unlike a naked quad you are not looking at the cells — you are looking at where four shapes are allowed to go.
What a hidden quad looks like
This is the hardest pattern on the board to see by eye, because nothing about the four cells stands out — they are usually the busiest cells in the unit, stuffed with candidates. The pattern lives in the shapes, not the cells: you have to ask, for each shape, which cells could still hold it.
The payoff is different from a naked quad. Nothing is removed from the rest of the unit — instead the four cells themselves get cleaned up, which is what turns a crowded cell into a solvable one.
How to use a hidden quad
- List where each shape can go. Take a unit and, for each unsolved shape, note which cells could still hold it. You are looking for shapes with two, three or four possible homes — a shape with five is no use here.
- Look for four shapes sharing four cells. Combine four of those shapes and count the cells they use in total. If four shapes between them touch exactly four cells, you have a hidden quad.
- Rule out the smaller version. If three of those shapes already fit in only three cells, that is a hidden triple and the fourth shape is a passenger. Apply the triple instead — it is a cleaner claim about a smaller set.
- Clear the four cells. Delete every candidate from those four cells except the four quad shapes. Those cells are now much tighter, and one of them is often a naked single or part of a fresh pair.
Hidden quads follow the same logic as hidden pairs and hidden triples, just wider. If you find them hard to see, it is worth getting quick at hidden pairs first — the search habit is identical, and at size two it is fast enough to practise constantly.
Hidden quads vs naked quads
The two are the same fact told from opposite ends. If four shapes are locked into four cells, then the other cells of that unit hold only the remaining shapes — which is a naked quad among them. So a unit with a hidden quad always has a naked quad too, and vice versa. Which one you spot depends on how many cells are still open: with five or six open cells the naked version is smaller and jumps out, but in a wide-open unit the hidden version needs less counting.
Find Hidden Quads in your own grid
Paste a puzzle and this page will look for Hidden Quads in it — the same pattern shown above, in your grid instead of ours. Nothing is uploaded; it runs in your browser.
Frequently asked questions
Why bother, if a naked quad says the same thing?
Because they are not equally easy to see. A hidden quad in a unit with eight open cells implies a naked quad among the other four — but finding that naked quad means checking every combination of four cells, while the hidden one only asks where four shapes can live. Whichever side has fewer things to count is the one to search.
Do the four cells need to be empty of other candidates?
No — the opposite. Extra candidates in those cells are exactly what a hidden quad removes. If the four cells held nothing but the four shapes already, the pattern would be a naked quad and there would be nothing left to strike.
How do I avoid confusing it with a hidden triple?
Count the cells before you commit. If any three of your four shapes already fit in only three cells, it is a hidden triple — apply that instead, since it says more with less. A genuine quad needs all four shapes to be involved in reaching the fourth cell.