Sudoku notes, also called pencil marks, record the numbers that could fit in an empty square. They are possibilities, not answers. A good note set helps you eliminate options without relying on memory.
Find the candidates for one square
Look at every number already in the square’s row, column and 3 × 3 box. Cross those numbers off the list 1–9. The numbers left are its candidates.
If the row rules out 1, 2 and 3, the column rules out 4 and 5, and the box rules out 6 and 9, the candidates are 7 and 8. Write both as notes; neither is confirmed yet.
Use the basic Sudoku rules to check each candidate. Do not copy another square’s notes without checking its row, column and box.
Keep notes up to date
Whenever you place a confirmed number, remove that candidate from every other empty square in the same row, column and box. Those squares can no longer contain it.
Do not remove the number from unrelated parts of the board. If an app changes some notes automatically, still review the affected squares yourself. Notes behaviour can vary between apps and versions.
Turn notes into answers
A single candidate
If a square has only one possible number left, that number is its answer. This is often called a naked single. It only works if your candidate list was complete and correct.
A single place
A row might have several empty squares, but only one can contain 4. Even if that square has other notes, 4 must go there. This is a hidden single. You can use the same idea in a column or box.
Avoid common mistakes
- Incomplete notes: one written candidate does not prove there is only one possibility.
- Old notes: update them after every confirmed entry.
- Empty candidate lists: recheck your notes and earlier entries before continuing.
- Guessing from notes: two candidates mean more deduction is needed.
Use the notes mode in Sudoku – Classic Puzzle to practise. Take one square at a time, and return to normal entry mode only when you have established an answer.