Candidate Notes in Sudoku: How to Use Them
Learn how to write and maintain Sudoku candidate notes so every later technique has reliable evidence.
Guided example
See the Candidate Notes in Sudoku: How to Use Them on this board
Read this as one teaching unit: first identify the active unit, then the marked pattern cells, then the candidates removed or maintained.
Quick answer
Candidate notes in Sudoku are small numbers written in empty cells to track every digit that could still legally fit. Good notes include only numbers not already blocked by the cell's row, column, or 3×3 box, then get updated after each placement or elimination.
How to spot it
Follow the board in order
- 1 Choose an unsolved cell and inspect its row, column, and 3×3 box.
- 2 Remove every number already present in those three areas.
- 3 Write only the remaining legal digits as candidates for that cell.
- 4 Update affected notes whenever a number is placed nearby.
What is candidate notation?
Candidate notes are the small numbers you write inside empty Sudoku cells. They are also called pencil marks. Each note means, “this number is still possible here.”
Candidate notes are not guesses. They are a compact record of the legal options left after the row, column, and 3×3 box are checked.
When to look for it
Use candidate notes when direct scanning stops producing easy placements. They become essential for techniques such as Hidden Single, Locked Candidates, Naked Pair, Hidden Pair, and X-Wing.
Beginners should add notes slowly at first: pick one empty cell, rule out the obvious numbers, then write only the remaining candidates.
Example walkthrough
This walkthrough uses a real bundled app puzzle, not a synthetic filled-candidate board. Look at row 2: the visible notes are different because each empty cell sees different row, column, and box constraints. For example, r2c8 keeps 2589 while nearby row cells keep narrower sets such as 35 or 139. That is the point of candidate notes: they record local evidence so the next technique is visible.
Why accurate notes work
Every Sudoku technique depends on the same rule: each number appears once per row, column, and box. Candidate notes make that rule visible. When the notes are complete and current, patterns become much easier to recognize.
Related concepts
Candidate notes are the bridge between beginner singles and candidate-elimination techniques. Single Candidate uses a cell with one note left, Hidden Single uses a number that appears in one place, and Locked Candidates use notes that are confined to one line or box.
Common mistakes
Do not write candidates from memory without checking all three constraints. Also avoid leaving stale notes after a placement; one forgotten pencil mark can make a later Naked Pair or Locked Candidate look valid when it is not.
FAQ
What are candidate notes in Sudoku?
Candidate notes, also called pencil marks, are the possible numbers that can still go in an empty cell.
When should I start using candidate notes?
Start using them when scanning no longer reveals easy singles or when you want to practice candidate-based techniques accurately.
Should candidate notes include every possible number?
They should include every legal possibility for the cell, but not numbers already blocked by the row, column, or box.
Do bad notes cause wrong Sudoku moves?
Yes. Missing or extra candidates can hide singles, create false pairs, or make eliminations look valid when they are not.
In this lesson
- Board example: see the pattern first.
- Walkthrough: connect each highlight to the rule.
- FAQ: check edge cases after the move is clear.
Practice this technique in Sudoku Coach
Read the pattern, then practice it step by step with guided hints that explain why the move works.
Related Sudoku techniques
Single Candidate in Sudoku: How to Spot It
Learn the Single Candidate Sudoku technique: check one cell's row, column, and box until only one legal number remains.
Hidden Single in Sudoku: How to Spot It
Learn the Hidden Single Sudoku technique: when a number has only one possible position in a row, column, or box.
Locked Candidates in Sudoku: How to Spot Them
Learn the Locked Candidates Sudoku technique: when candidates are trapped in one line or one box and can be removed elsewhere.