Guide

Stuck on a Sudoku? What to try next

Every solver hits a point where no move shows itself. It almost never means you have to guess. It means the next move is one you haven't looked for yet, and there is an order to looking.

Ninesmith Sudoku · November 12, 2026

Why you get stuck

When no move is in sight, it is one of three things:

• You missed a single. A cell with one digit left, or a digit with one place left. This is the most common by far.
• The puzzle needs a technique you aren't using yet, such as a pair or locked candidates.
• A digit placed earlier is wrong, and the board has quietly stopped making sense.

The steps below check them in that order, cheapest first. Whenever a step finds something, go back to the first one: every new digit can open moves you have already scanned past.

The examples all come from one puzzle. Rows count from the top, columns from the left, and boxes in reading order: box 1 is the top left, box 5 the centre, box 9 the bottom right.

1. Scan again, digit by digit

Most stalls end here. Pick a digit and a box that doesn't have it yet. For each empty cell in the box, check whether that digit already sits in the cell's row or column. If it does, the cell is out. When only one cell is left, the digit goes there.

This is a Hidden Single. Hidden, because the cell itself could still take 4, 8 or 9; it is the 9 that has nowhere else to go. Rows and columns work the same way: look for a digit with only one free spot left in the line.

When you're stuck, the trick is to be systematic instead of wandering the board. Take 1 through every box, then every row and column, then 2, and so on up to 9. Start with the digits the board has most of, because they shut the most cells.

2. Look where the board is fullest

Two moves hide where the board is crowded. The first is the last empty cell: a row, column or box with eight digits in it already has its ninth decided.

The second starts from a cell instead of a digit. Pick an empty cell where a full row and a full column cross, and gather every digit it can see in its row, its column and its box. If that is eight different digits, the ninth is the only one that fits. This is a Naked Single.

You can finish this position on the board on the home page, and ask the Smith there if you get stuck.

3. Write down the candidates

Easy puzzles fall to singles alone. Harder ones run out of them, and from there on you need notes: write each empty cell's candidates, the digits not yet in its row, column or box, small inside the cell.

Notes only help while they are current. Each time you place a digit, strike it from the notes in its row, column and box. A cell down to one note is a Naked Single. A digit left in the notes of only one cell of a row, column or box is a Hidden Single.

In Ninesmith, the Notes button writes them one at a time, and Auto Notes fills in every empty cell at once.

4. Look for pairs

If two cells in the same row, column or box have the same two candidates and nothing else, those two digits belong in those two cells. You don't know yet which goes where, but no other cell in that row, column or box can have either, so strike both from the rest of it. This is a Naked Pair.

The same idea works with three cells. If three cells in a row, column or box share only three candidates between them, that is a Naked Triple, and those three digits come out of the rest of it. Each cell needn't hold all three; together they just can't hold a fourth.

It also works the other way round. If two digits can only go in the same two cells of a row, column or box, that is a Hidden Pair, and every other candidate comes out of those two cells. A Hidden Triple is the same with three digits and three cells.

5. Look for locked candidates

Sometimes a digit, inside one box, can only sit in one row or one column. You don't know the cell yet, but you know that row's copy of the digit is in this box. So the rest of the row, outside the box, can't have it.

It works the other way too: if a digit in a row or column can only go in cells of one box, strike it from that box's other cells.

6. Patterns for hard puzzles

If none of the above moves anything, the puzzle is built around one of the bigger patterns. Like locked candidates, each one only strikes candidates; the singles it opens do the rest.

X-Wing

In two rows, a digit can only go in the same two columns. The four cells make a rectangle, and the digit takes two opposite corners, so it comes out of the rest of both columns. It works the same with columns in place of rows.

Swordfish

The same idea across three rows and three columns: if a digit can only go in the same three columns in three rows, it comes out of the rest of those columns.

Y-Wing

Three cells with two candidates each: AB, AC and BC. The AB cell sees both others. If it is A, the AC cell must be C; if it is B, the BC cell must be C. One of them is C either way, so C comes out of every cell that sees both.

Unique Rectangle

This one leans on the puzzle having one solution. Four cells at the corners of a rectangle, across two boxes, can't all hold only the same two candidates, because then the two digits could swap and there would be two solutions. If three corners hold only those two, the fourth must take one of its other candidates, so those two come out of it.

The Pattern Trainer in Ninesmith drills these in sessions of 30 to 90 seconds, from singles, pairs and triples up to X-Wing, Swordfish, Y-Wing and Unique Rectangle. Singles to triples are free. The four advanced techniques are in Premium, and the first scenario of each is free too.

Still stuck? Check for a mistake

If you have worked through all of it and the board still won't move, or a cell ends up with no candidates at all, suspect a mistake.

• Look for a repeat. Go box by box, then row by row and column by column. A digit that appears twice means one of the two is wrong.
• Look for a cell with no candidates. If an empty cell can't take any digit, something placed earlier is wrong.
• Check your notes against the board. A note you forgot to strike makes false pairs and false singles.
• Go back to your last guess. If you placed a digit because it fit rather than because it had to go there, start looking there.

How to get stuck less often

• Scan in the same order every time. A fixed order finds what a wandering eye skips.
• Place only what you can explain. A digit that fits is not a digit that must go there.
• Write notes before you're desperate. On harder puzzles, write them as soon as the easy singles dry up.
• Learn one technique at a time. Drill it until you spot it without looking for it, then add the next.
• Play a little every day. A puzzle at your level that you finish teaches more than a hard one you abandon.

Questions