How to use notes in Sudoku

Notes are small numbers that record the possible values for an empty cell. They help you remember what you have ruled out without committing to an answer.

You may also see them called candidates or pencil marks. A note means “this number could fit.” It does not mean the number is definitely correct.

Build a candidate list

Choose an empty cell. Begin with the numbers 1 to 9, then remove any number already present in the cell’s row, column, or 3 × 3 box.

Suppose those three groups contain 1, 2, 4, 6, 7, and 8 between them. The cell’s candidates are 3, 5, and 9. Write those as notes and continue looking for more information.

If there is one candidate left, you have a move. If there are no candidates left, recheck your earlier entries or your notes. An empty candidate list is a reason to investigate, not to guess.

Update the affected cells

After entering a number, remove it from notes in the same row, column, and box. Leave notes elsewhere alone unless another deduction rules them out.

Some apps can update candidate notes automatically. If you use that option, first understand whether it maintains all candidates or only removes numbers from notes you entered yourself. The exact behavior varies by app.

Avoid mixing notes and answers

On paper, keep notes small and answers larger. In an app, look for a clear indicator that notes mode is on. Check that an accidental mode change is easy to correct before you start a long puzzle.

You don’t have to fill every empty cell with notes immediately. A beginner can start with cells that have only two or three possibilities, provided they remember that an unmarked cell may still allow other numbers. For techniques that rely on a complete candidate picture, make the notes complete first.

Keep notes readable

Notes have less room than final answers. When choosing a large print app, inspect the candidates separately from the main digits. Being able to read a finished number does not guarantee that a cluster of small notes is comfortable to read.

Try our app checklist. Then use your notes to find single-candidate and hidden-single moves.