Hidden Singles: Finding the Only Home for a Digit
Pick a 3×3 box that is nearly full — say it still needs a 4, a 7, and an 8. Now ask a different question than usual: not "what can go in this empty cell?" but "where can the 4 go in this box?" Check each empty cell against its row and column. If two of the three empty cells already see a 4 somewhere along their row or column, the 4 has only one legal home left. That cell is a 4 — a hidden single — even though, on its own, it might still look like it could take a 7 or an 8.
That shift in viewpoint is the whole technique, and it is the one that gets you moving again the moment naked singles dry up. A naked single asks about a cell; a hidden single asks about a digit. Both are pure logic with no guessing, and strong solvers flip between the two without thinking about it.
Naked single vs. hidden single
The two techniques are mirror images. A naked single is a cell with only one candidate left. A hidden single is a digit with only one candidate cell left in a unit — a row, a column, or a box. The hidden single is "hidden" because the winning cell often still shows several pencil marks; nothing about it screams "I must be a 4" until you count where else the 4 could go and find nowhere.
This is exactly why beginners miss them. If you only scan for cells that are down to one mark, you will walk straight past a hidden single sitting in a cell that still shows three candidates.
A worked example
Take a box that needs 4, 7, and 8, with three empty cells: top-left, middle, and bottom-right. The 7 is the digit to chase. Suppose the top-left cell shares its row with a 7 already placed elsewhere, and the middle cell shares its column with another 7. Both are ruled out for 7. That leaves only the bottom-right cell, so it must be 7 — locked in, even though it also looked like it could be a 4 or an 8.
Place the 7, and the box is down to 4 and 8 in two cells, which usually resolve as naked singles or with one more row and column check. One hidden single often unzips the rest of a box.
Scan by digit, not by cell
To find hidden singles on purpose, walk the digits 1 through 9 and, for each, look across the units where it is still missing and ask where it can legally land. A practical order:
- Pick a digit already placed many times across the grid — it casts the most constraints, so it is the most likely to be pinned to a single cell.
- Scan boxes first: nine cells in a compact area means a digit is often confined quickly.
- Then scan the rows and columns for that same digit before moving on to the next number.
Why pencil marks can hide them
Automatic pencil marks make naked singles obvious but can camouflage hidden singles, because the winning cell still shows multiple marks. The fix is to read the marks by digit: scan a unit for every cell containing a 4, and if exactly one cell in that unit has a 4 penciled in, that cell is the 4 — clear the other marks and place it.
In Sukuro the learning levels colour each digit, which makes this sweep faster: you are hunting for the only cell in a row, column, or box still carrying a given colour. The Hint button will also surface a hidden single and name the unit it is hiding in when you get stuck.
Practice it now
Load an easy-to-medium board and force yourself to alternate: clear the naked singles, then deliberately scan digit by digit for hidden singles before doing anything else. After a few puzzles the box-by-box "where can this digit go?" sweep becomes a reflex, and medium Sudoku stops feeling stuck.