What to do when you are stuck

What it is

Every player gets stuck. You have been filling cells happily, and then nothing. You look at the grid and no move seems possible. This guide is a routine for that moment. It will not give you a single trick, but it will get you moving again far more often than staring at the grid.

The key thing to remember: every puzzle on sudoku.ie has exactly one solution and can be solved by reasoning. If you are stuck, a move exists. You just have not found it yet.

Step 1: check for mistakes

Before hunting for a clever move, make sure the grid is right. A single wrong digit can make the puzzle seem impossible. Look for:

• the same digit twice in any row, column or box,
• an empty cell with no candidates left,
• a digit that has nowhere left to go in some row, column or box.

Any of these means something went wrong earlier. On sudoku.ie, you can use undo to step back to before the mistake.

Step 2: tidy your notes

Stale pencil marks are the most common reason people get stuck. Go through each digit you have placed and make sure it has been removed from the notes of every cell in its row, column and box. You will often find a cell that was down to one candidate all along.

If you have not been keeping full notes, now is the time to fill them in. Many moves only become visible once every candidate is written.

Step 3: work through the techniques in order

Go from simplest to hardest. Do not jump to an X-wing before you have checked for singles.

1. Singles. Look for naked singles (a cell with one candidate) and hidden singles (a digit with one place in a row, column or box). Check rows and columns, not just boxes.
2. Pointing and claiming. For each box, see if any digit is confined to one row or column (pointing). For each row and column, see if any digit is confined to one box (claiming).
3. Pairs. Look for two cells with the same two candidates (naked pair), and two digits sharing the same two cells (hidden pair).
4. Triples, quads and X-wings on Hard days.
5. Wings and swordfish on Expert days.

After every elimination, go back to step 1 of this list. Advanced moves usually unlock a run of singles.

A worked example

Say you are stuck on a Wednesday Medium. Your notes are full, and no cell has one candidate. You check box 7 for pointing and find that 2 can only go in row 8, column 1 or row 8, column 3. That points along row 8, so you remove 2 from row 8, column 6. That cell had notes of 2 and 9, so it is now 9.

Placing that 9 removes 9 from the rest of column 6. Row 2, column 6 had 4 and 9, so it becomes 4. One small pointing move has set off a chain.

Step 4: change your view

If you still cannot see anything:

• Pick one digit and look only at that digit across the whole grid. Patterns like the X-wing become much clearer this way.
• Look at the unit with the fewest empty cells. It often has the simplest answer.
• Take a break. A cup of tea and five minutes away really does help. Fresh eyes spot things tired ones miss.

Step 5: ask for a hint

The hint button on sudoku.ie does not just fill in a cell. It explains the next logical step and names the technique it uses. That way you learn the move rather than just getting past it. There is no shame in a hint, and using one is a good way to learn a technique you have not met yet.

A common mistake

The most common mistake when stuck is guessing. It is tempting to pick one of two candidates and see what happens. But a wrong guess can take many moves to show itself, and by then the grid is in a tangle. Reasoning is always possible, so trust it.

When you will need it

At every level, but especially when you step up a difficulty: from Easy to Medium, or from Medium to Hard. Those are the moments when your current toolkit runs out and a new technique is waiting to be learned.