Hidden singles in sudoku
The hidden single flips the question. A naked single asks what a cell can hold; the hidden single asks where a digit can go. Inside one row, column or box, a digit down to a single possible cell claims it, whatever else that cell might have accepted. Most of an easy solve runs on this move, and it stays busy all the way up the ladder.
The pattern
Watch the 4s work on the middle-left box. One placed 4 bars the box’s top row, another bars its bottom row. A third stands in the first column, a fourth in the third column, and the 9 and 1 fill two cells outright. Count what survives for a 4: one cell.
Here is what makes the single hidden: that surviving cell is no naked single. Its own row and column are nearly empty, and cell-first reasoning would shrug at it. The force comes entirely from the digit’s side of the question. The box must contain a 4, and every alternative is barred, so the 4 goes there even though the cell looked wide open.
How to spot them
Scanning delivers most hidden singles without ceremony: sweep a digit, watch a box lose cells, place when one remains. The deliberate version works unit by unit instead. Pick a row, column or box that has stalled, and for each digit it still needs, count the possible homes. A count of one is a placement, whatever the cell’s other options.
Boxes give the quickest wins because rows and columns cut through them at right angles, stacking eliminations fast. Busy digits make the best targets: a digit placed five times has four units left to serve, each of them under pressure from the copies already down.
Near misses
The claim rests on every alternative being barred. Remove the column blocker at the top and a second cell reopens:
Two homes is not a placement, and pretending otherwise is where easy solves go wrong. It is still progress. Two homes in one box form the seed of locked candidates when they share a line, and the exactly-two situation itself becomes the strong link that powers the skyscraper tiers above. The counting habit you build here is the one the whole ladder runs on.
Where it fits
Every easy puzzle here is graded to fall to scanning and the two singles, no pencil marks required. When you catch yourself counting homes without thinking about it, easy has taught you what it came to teach, and medium is ready with the first candidate techniques.
Frequently asked questions
What is a hidden single in sudoku?
A digit that has exactly one possible cell left within a row, column or box. That cell might accept other digits on its own, and none of that matters: the unit needs this digit somewhere, and only one cell can take it.
Why is it called hidden?
Because the cell hides the fact. Looked at cell-first, it seems open to several digits and nothing forces a choice. The single only appears when you count homes for the digit across the whole unit, which is why digit-first thinking finds moves that cell-first thinking misses.
How do you spot hidden singles?
Scanning finds most of them: sweep a digit, watch a box lose cells to its rows and columns, and place when one cell survives. For the rest, pick a unit and count homes per missing digit. A count of one is a placement.
Are hidden singles harder than naked singles?
A little, and they repay the effort. Naked singles need a crowded neighborhood, so they appear less often. Hidden singles fire all game long, and our easy puzzles are graded so that scanning plus both kinds of single carries you start to finish.