How to solve hard sudoku: pointing pairs, X-wings, and more
Most casual sudoku players hit a wall around the hard tier: the “scan rows and columns for missing numbers” technique that solves easy and medium boards stops working. The board sits there with no obvious singles, no obvious pairs, no progress. Here’s what actually unlocks it.
Step zero: pencil-mark properly
You can’t solve hard sudoku without candidate marks. Every advanced technique works by spotting patterns in the candidatesof empty cells, the small numbers showing which digits could still go there. If your board isn’t fully pencil-marked, you’re flying blind.
On Melio Sudoku, turn on auto-candidates (or fill them yourself with the notes mode). From here on, every technique assumes you can see candidates in every empty cell.
Hidden singles, the first technique past scanning
A “naked single” is a cell that has only one candidate left, those solve themselves. A hidden single is sneakier: a cell whose candidate appears only once in its row, column, or 3×3 box, even if the cell itself still shows several options.
Example: Row 3 has empty cells at columns 2, 5, and 7. Cell (3,2) has candidates {2, 5, 9}; (3,5) has {5, 7}; (3,7) has {5, 9}. The candidate 2 appears in row 3 only at (3,2). That cell must be a 2, even though by itself it looks like it could be 2, 5, or 9.
Hidden singles unlock most “hard” puzzles entirely. Always check them first before reaching for fancier techniques.
Naked pairs and triples
When two cells in the same row (or column, or box) have exactly the same two candidates, you’ve got a naked pair. Whatever the two digits are, they must fill those two cells in some order. That means you can eliminate those digits from every other cell in that unit.
Example: In column 4, cells (1,4) and (6,4) both have candidates {3, 8}. You don’t know which is 3 and which is 8, but you know neither 3 nor 8 can appear anywhere else in column 4. Erase 3 and 8 from the other empty cells in that column. Often this exposes a hidden single or a new naked pair.
Naked triples are the same idea with three cells sharing three candidates: e.g., three cells with candidate sets {2,5}, {2,7}, {5,7} in the same row. Together they lock down 2, 5, and 7, eliminate those three from every other cell in the row.
Naked pairs + triples deep-dive → for the full pattern catalog (including the hidden-pair sibling and the bivalue triple combinations).
Pointing pairs and box-line reduction
Pointing pairs (also called intersection removal) are probably the highest-leverage technique to learn. They work between a box and a line.
Pointing pair: If a candidate appears in a single box only within one row (or one column), that candidate must end up in that row inside that box which means you can eliminate the candidate from the rest of the row (the cells outside the box).
Example: In the top-left 3×3 box, candidate 7 appears only in cells (1,2) and (3,2), both in column 2. Whichever of those two ends up being 7, you know 7 must be in column 2 within that box. So you can erase 7 from every other cell in column 2 outside that box (rows 4-9). Often this opens up a hidden single in another box.
Box-line reductionis the reverse: if a candidate in a row appears only inside one box, you can erase that candidate from the other rows’ cells in that same box.
The X-wing, when nothing else works
The X-wing is the first technique most players consider “advanced.” It’s actually simple once you see it. Look for the same candidate appearing exactly twice in two different rows, and in thesame two columns.
Example: Candidate 4 appears in row 2 only at columns 3 and 7. Candidate 4 also appears in row 8 only at columns 3 and 7. You’ve got an X-wing on the digit 4.
Here’s the logic: in row 2, the 4 is either at column 3 or column 7. Same in row 8. Suppose the row-2 4 is at column 3, then the row-8 4 must be at column 7 (otherwise column 3 would have two 4s). And vice versa. Either way, one of columns 3 and 7 has its 4 in row 2, and the other has its 4 in row 8. No other row in those two columns can contain a 4.
So: erase the candidate 4 from every other cell in columns 3 and 7. That’s often the unlock for the entire puzzle.
X-wings work equally well across columns (find a candidate that appears in only two columns, and those columns are in the same two rows). Same logic, transposed.
When to use which technique
Roughly the order you should check, from cheapest to most expensive in attention:
- Naked singles, cells with one candidate. Always free.
- Hidden singles, candidates appearing only once in a unit. Free if you’re pencil-marked.
- Naked pairs / triples, eliminate aggressively whenever you spot one.
- Pointing pairs / box-line reduction, the biggest single technique past hidden singles.
- X-wing, when intersection-removal stops producing.
- Swordfish, XY-wing, coloring, for the very hardest puzzles. We’ll cover these in a follow-up guide.
Most expert-tier sudoku puzzles on Melio (and elsewhere) are solvable with techniques 1-5. Only the “extreme” tier reliably needs swordfish or further.
The mistake to avoid: guessing
When you hit a wall, the temptation is to guess: pick a cell with two candidates, try one, see if it leads to a contradiction, back out if it does. This works, but it’s slow, error-prone, and you don’t learn anything.
Almost any “hard” sudoku has a deductive solution. You just haven’t spotted it yet. Walking away for five minutes and coming back fresh produces breakthroughs more reliably than grinding the same scan repeatedly.
On Melio, the conflict-highlighting and pencil-mark UI are designed to make these techniques visible. The board auto-highlights related cells when you select one, use that to see the candidate landscape, not just the digits you’ve already placed.
Practice deliberately
Reading about techniques only goes so far. The skill is pattern recognition, and that comes from solving puzzles where you intentionally use the technique you just learned.
Play a few expert-tier Melio Sudoku puzzles trying to spot pointing pairs specifically. Then a few looking only for X-wings. After 10-20 puzzles per technique, the patterns start to jump out without you searching.