Sudoku Solving Techniques: From Naked Singles to X-Wing
Every valid Sudoku has exactly one solution and can be reached by pure logic — no guessing required. The trick is knowing which technique to reach for, and when. This guide walks through them in the order they actually become necessary, from the moves you'll use on every puzzle to the ones that crack a stubborn Hard grid.
First, the ground rule
Each of the nine rows, nine columns and nine 3×3 boxes must contain the digits 1–9 exactly once. Every technique below is just a smarter way of asking, "given what's already placed, where can this digit legally go?" On our Sudoku board, every puzzle is generated and then verified to have a single unique solution, so you can trust that logic alone will always finish it.
1. Naked singles — the workhorse
A naked single is a cell where eight of the nine digits are already ruled out by its row, column and box, leaving exactly one legal value. These are the bread and butter of Easy puzzles and the mopping-up of every puzzle. Before anything clever, always sweep the board for cells with only one candidate left.
2. Hidden singles — the most missed easy move
A hidden single is sneakier. A cell might have several candidates, but if it is the only cell in its row, column or box where a particular digit can go, that digit must belong there — even though the cell "looks" undecided. Beginners miss these constantly. The habit that finds them: pick a digit, say 7, and for each box ask "where can a 7 possibly go here?" If only one cell survives, you've found a hidden single.
Pencil-mark habit: writing tiny candidate lists in each empty cell turns hidden singles and every later technique from guesswork into reading. If you're stepping up to Medium and Hard, start pencil-marking — it's the single biggest jump in solving power.
3. Pointing pairs — link a box to a line
Once you're pencil-marking, look inside a box for a digit whose only candidates sit in a single row or column of that box. That digit must land somewhere on that line within the box, which means it cannot appear in that same row or column outside the box. You get to erase candidates elsewhere — often unlocking a fresh naked or hidden single. This "box-line reduction" is usually the first technique that separates a Medium from an Easy.
4. Naked pairs and triples — shared exclusivity
If two cells in the same unit (row, column or box) both hold exactly the same two candidates — say {4, 8} — then 4 and 8 are "reserved" for those two cells. No other cell in that unit can be 4 or 8, so you can strike those candidates out everywhere else in the unit. The same idea extends to three cells sharing three candidates (a naked triple). Clearing these opens up the board without placing a single digit directly.
5. The X-Wing — your first real "aha"
The X-Wing is where Sudoku starts to feel like magic. Look for a digit — say 5 — that in two different rows has candidates in exactly the same two columns, and nowhere else in those rows. Those four cells form the corners of a rectangle. Whatever way the 5s eventually fall, they will occupy one diagonal of that rectangle, which means the two columns are fully accounted for. So you can remove 5 as a candidate from every other cell in those two columns. The same works with rows and columns swapped. It looks like sorcery the first time and becomes routine the tenth.
Choosing the right tool at the board
You don't apply these at random. A reliable escalation order:
- Sweep for naked singles.
- Hunt for hidden singles, one digit at a time.
- Pencil-mark, then look for pointing pairs.
- Clear naked pairs / triples.
- Only when truly stuck, search for an X-Wing.
Most of the time you'll never need step 5 — but on our Hard puzzles (around 26 clues) it's occasionally the key that turns a dead board back into a chain of easy placements.
A word on mistakes and hints
Because logic always suffices, a "mistake" in Sudoku is really a reasoning error, not bad luck. Our board caps you at three mistakes and offers three hints, which makes it a good training ground: when you're tempted to guess, stop and ask which technique above you haven't tried yet. Save the hint for when you've genuinely exhausted them — you'll learn far more.
Warm up on Easy to drill singles, then move to Medium and Hard to practice the pairs and the X-Wing. The techniques stack, and once they're second nature, even the sparse grids fall in a steady rhythm.
More guides: Minesweeper number patterns and Triple Tile strategy.