Most people who say they're bad at Sudoku are describing the same moment. You open a puzzle, you look at eighty-one squares, and nothing announces itself. There's no obvious place to push. So you guess, and a guess quietly poisons everything downstream, and forty minutes later two 7s are sitting in the same row and you can't tell when it went wrong.
That's not being bad at Sudoku. That's not having a first move.
Here's the first move. It's the technique everything else is built on, and it takes about two minutes to learn.
The thing you're looking for is called a naked single: a square whose row, column and box have already ruled out every number but one.
Look for the square that has run out of options
Every empty square has to hold a number from 1 to 9. Three things restrict it: its row, its column, and its 3×3 box. A number already sitting in any of those three is out.
So instead of asking "what goes here?", ask the smaller question: what can't go here? Cross those off. Sometimes you'll cross off eight of the nine, and whatever survives isn't a guess. It's the answer. There is nothing else it could be.
The numbers still standing for a square are its candidates. That word shows up in every Sudoku explainer going, and it only ever means "what's still allowed here."
Let's do one.
The highlighted square is dead centre. Its row, its column and its box are shaded with it — that's the neighbourhood we care about, and it's the only neighbourhood that matters.
Now cross off, one region at a time, and keep a running count as you go. I'll list what each region already holds from low to high, rather than in the order they sit on the board — it's easier to check against a 1-to-9 list that way.
Its row already contains 1, 3, 4 and 8. Those four are gone. Running total: four.
Its column already contains 1, 2, 6, 7, 8 and 9. That's six numbers, but the row already took 1 and 8, so only four of them are new: 2, 6, 7 and 9. Running total: eight.
Its box already contains 2, 3, 6 and 8 — and every one of those is already crossed off. The box adds nothing here. Running total: still eight.
That last step is the one that makes people think they've gone wrong. A region that contributes nothing feels like a failed step. It isn't; it just means the row and column got there first. Work the box anyway, every time. On another square it'll be the region that finishes the job.
Eight numbers eliminated — 1, 2, 3, 4, 6, 7, 8 and 9. One survivor.
It's a 5. Not probably a 5. Not a 5 unless something later contradicts it. The only number that can legally sit there.
A guess is a number you'd have to take back. This isn't one. That difference is the whole game.
The jargon, deflated
The name came at the top: naked single — a square whose candidates have been stripped down until one is left standing. The name is worse than the idea.
You'll meet a lot of names like that. Hidden singles, pointing pairs, box-line reduction, X-wing. Every one of them is a rule about crossing things off. None of them requires arithmetic; classic Sudoku has nine symbols and no addition anywhere in it. They could be nine colours or nine animals and the puzzle would work identically.
If someone told you you needed to be good at maths for this, they were wrong, and it probably cost you a few years of a decent hobby.
Why the shading is the whole trick
Notice what actually did the work up there. Not cleverness — the highlight. Once the square's row, column and box were shaded together, the eliminations were just reading. Every number in the shaded region is a number that can't be in the middle.
That's why the app shades a square's row, column and box the moment you touch it. It isn't decoration. It's the technique, drawn.
This is also the line we drew when we built it. There's no hint button. It won't solve a square for you and it won't nudge you toward the next move, because the moment it does, the puzzle stops being yours and the satisfaction goes with it.
What it will do is the clerical work. Every key on the pad shows how many of that digit are still unplaced, so you can see at a glance which numbers are nearly finished.
None of that is thinking. It's the bookkeeping a pencil makes you do by hand, and it's exactly the part worth automating. The thinking stays where it belongs.
Write it down — pencil marks are not cheating
Here's what actually happens without them. You work out that a square can only be a 2 or a 5, you don't write it down, you move on, and four minutes later you do the same elimination again. Most of what people call "being stuck" is time spent re-deriving things they already worked out once.
So the belief that writing candidates into the squares is a form of failure — that real players hold it all in their heads — has it backwards. Constructors don't hold it in their heads. The people who finish Expert puzzles in eleven minutes don't. Pencil marks aren't training wheels; they're the notation the technique is written in.
Write them small in the corner. Two numbers, three numbers, however many survive. And when a square comes down to one, you've found a naked single without having gone looking for one.
Three words, one idea, in case you've been wondering: candidates are what's still allowed in a square, pencil marks are those candidates written down, and Notes is what our app calls the button that writes them. Tap Notes first — it's a mode, and until it's on, a number you type goes into the square as a real answer rather than a small one in the corner.
The bookkeeping cost of this is the reason people avoid it. Place a 5 and you now have to walk the row, the column and the box rubbing out every 5 you'd pencilled in — and if you miss one, that stale mark will lie to you twenty minutes later.
That specific chore is worth handing to a machine, and it's what our app does. Place a number and it clears that square's marks and strips the digit from every square in the row, column and box automatically. Nothing goes stale. Nothing lies to you.
Your marks stay yours. Only the erasing gets done for you.
Now find one yourself
Same puzzle, one move further on — the 5 is placed. And placing it did exactly what the last section promised: it crowded its neighbours. There are now six naked singles sitting on this board.
Six, not one. Which means you can't get this wrong by finding a different square than I did. Any square whose row, column and box between them rule out eight of the nine numbers is a naked single, and six of them qualify. Find any one you like.
If you'd rather aim at a specific square, take the marked one below: bottom-middle box, top row, right-hand column.
Work the three regions in order — row, then column, then box — and keep the running count. Don't try to hold it all in your head at once; that's what pencil marks are for.
One warning, and it's the reason I laboured the point earlier. In the worked example the box added nothing. On this square it adds the last two eliminations. Skip it and you'll stop at six, find three numbers still standing instead of one, and conclude you've broken something. Work all three regions every time.
All six answers — open when you've had a go
Rows count from the top, columns from the left.
| Square | Answer |
|---|---|
| row 5, column 2 | 2 |
| row 6, column 4 | 9 |
| row 7, column 5 | 3 |
| row 7, column 6 — the marked one | 7 |
| row 7, column 9 | 4 |
| row 8, column 8 | 3 |
If you found one that isn't on this list, check it again — but check it against the rule, not against me. Eight numbers eliminated, one survivor. That's the whole test.
What to do with this
Next time you open a puzzle and nothing announces itself, don't scan for something interesting. Scan for something cornered. Squares sitting in a crowded row, a crowded column and a crowded box at the same time — the middle of a busy region is where singles hide.
They're not rare, either. Of the 300 puzzles that ship inside our app, 288 have a naked single somewhere in their solve path. This is not a special-occasion move; it's the everyday one.
It narrows the search rather than ending it. The most hemmed-in square on a board often comes down to two candidates rather than one, and then you move to the next most hemmed-in. That's not a failed attempt, it's the search working: two or three checks to find one square is still enormously faster than reading all eighty-one.
Find one. Place it. That placement crowds its own neighbours, which often produces the next single, and the next. A puzzle that looked inert five minutes ago starts falling over.
That's the loop. Everything else in Sudoku is a more elaborate way of doing what you just did.
Including the one that comes next. When the loop stops paying — you've been round the board twice and every empty square still has three or four numbers standing — you haven't hit a wall. You've run out of this question. There's a second one, and it's this one turned around: not which square has one number left, but which number has one square left. That's a hidden single, and it's the thing to learn second.
If someone you know says they're no good at Sudoku, this is the thing to send them. It's usually the only thing they were missing. When you want to know what comes after it, Sudoku techniques, in the order worth learning them lays out the whole path on one page.