Sudoku 
Classic
Sudoku
The standard 9×9 game with pencil mode, Auto Notes, hints, undo, and four difficulty levels.
Play SudokuSudoku pencil marks are small notes inside empty cells. They show which digits might still be possible after checking the row, column, and 3×3 box.
Different solvers arrive with different questions: some want a Sudoku game with notes, some want a quick definition, and some want a practical tutorial. This guide covers all three.
Quick answer
Quick answer
Pencil marks, also called notes or candidates, are tiny numbers you place in an empty Sudoku cell to track possible answers. On our games you can add them manually with pencil mode, or use Auto Notes to fill candidate marks for the current grid.
Play Sudoku with Pencil Marks
All of these games let you use manual pencil marks and Auto Notes. Start simple if you are learning, or move up when you want candidate patterns like naked singles, hidden singles, and pairs to matter.
Practice
Medium Sudoku
A good level for learning notes: easy moves still appear, but pencil marks begin to reveal useful patterns.
Play Medium Sudoku
Challenge
Hard Sudoku
Harder puzzles make pencil marks essential for keeping candidates tidy and avoiding guesses.
Play Hard SudokuWhat Are Pencil Marks?
Pencil marks are not part of the original puzzle. They are solving notes you add yourself. If a cell could be 2, 4, or 8, you mark 2, 4, and 8 in small type until the surrounding logic removes one or more of them.
They are also called candidates, notes, or candidate notation. On paper, players write them lightly so they can be erased. Online, the same idea appears as a notes mode or pencil mode.
Manual vs Auto Pencil Marks
Manual pencil marks are best when you want to think through the logic yourself. Toggle pencil mode, select a cell, and tap a digit to add or remove that note.
Auto Notes are faster. They scan the current grid and add every digit that is legal in each empty cell. This is useful when a puzzle gets busy, but you should still understand why those candidates are possible.
Using Notes in Our Sudoku Game
On our Sudoku boards, the pencil-mark feature is the Notes button. Use it when you want to enter small candidate numbers instead of a final answer.
- Select a cell: Click or tap an empty square in the grid.
- Turn on Notes: Press the Notes button. It stays active while you add candidate marks.
- Add candidates: Tap the number buttons for every digit that could still fit in that cell.
- Remove a note: While Notes is active, tap the same digit again to toggle that pencil mark off.
- Place a real answer: Turn Notes off, then tap a digit to enter it as the cell's answer.
- Use Start Notes: If you want a full candidate map, press Start Notes to auto-fill legal candidates for the current grid.
Tip
Use Start Notes when you are learning medium or hard puzzles. Use manual Notes when you want to practise the candidate logic yourself.
How to Add and Remove Pencil Marks
- Select an empty cell.
- Turn on pencil mode.
- Choose each digit that could still fit in that cell.
- Tap the same digit again to remove a note if it is no longer possible.
- Turn pencil mode off when you are ready to enter a confirmed answer.
When you place a real digit, notes in that cell disappear. Matching notes in the same row, column, and box are removed too, which keeps the grid cleaner as you solve.
Worked Example: Finding Pencil Marks for One Cell
Suppose an empty cell could contain any digit from 1 to 9. To find its pencil marks, remove every digit already seen by that cell.
- The row already contains 1, 2, and 5, so those digits are impossible.
- The column already contains 3 and 8, so remove those too.
- The 3×3 box already contains 4 and 9.
The only digits not eliminated are 6 and 7, so the pencil marks for that cell are {6, 7}. If a later move places a 6 in the same row, column, or box, remove 6 from this cell and the answer becomes 7.
How to Use Pencil Marks While Solving
- Start after obvious moves: On easy puzzles, scan for clear placements before filling lots of notes.
- Update after every placement: A stale note can make a wrong candidate look possible.
- Look for singles: A cell with one note left is a naked single. A digit that appears as a note in only one place in a row, column, or box is a hidden single.
- Look for pairs: Two cells in the same unit with the same two notes can often eliminate those notes from nearby cells.
- Keep the grid readable: Notes are a tool, not a decoration. Remove candidates as soon as the logic rules them out.
Full Notes vs Partial Notes
Full notes means filling every empty cell with every legal candidate. This is best for learning, for hard puzzles, and for spotting patterns reliably. The trade-off is visual clutter.
Partial notes means marking only useful candidates, such as a digit you are tracking across one row or box. This keeps the grid cleaner, but it requires more mental bookkeeping. Beginners usually do better with full notes on medium puzzles, then move toward partial notes as they get faster.
When Not to Use Pencil Marks
You do not need pencil marks for every Sudoku puzzle. On easy grids, first look for obvious placements with scanning, crosshatching, and simple elimination. If the grid is still giving you clear answers, adding notes can slow you down and make the board look busier than it needs to be.
A good rule is: start pencil marks when you are rechecking the same rows and boxes without finding a new number.
Techniques That Use Pencil Marks
Once your notes are accurate, several solving techniques become much easier to see. Start with naked singles and hidden singles, then move on to naked pairs and hidden pairs. These patterns are the main reason pencil marks matter in medium and hard Sudoku.
Common Mistakes
| Mistake | Why it hurts | Better habit |
|---|---|---|
| Adding notes too early | The grid becomes noisy before simple placements are found. | Scan first, then add notes when progress slows. |
| Not removing old notes | Old candidates lead to false patterns. | Clean affected rows, columns, and boxes after every placement. |
| Using Auto Notes without checking | You may follow marks without understanding the logic. | Use Auto Notes as a speed tool, then verify key eliminations. |
Frequently Asked Questions
Pencil marks are small candidate notes inside empty cells. They show which digits could still fit based on the current row, column, and box.
Yes. Pencil marks, notes, and candidates usually mean the same thing in Sudoku.
Beginners can use them, but not on every easy puzzle. Try scanning first, then add notes when you stop finding obvious moves.
Auto Notes fills each empty cell with the digits that are currently legal. It saves time, especially on medium and hard puzzles.
In pencil mode, tap an existing note to remove it. When you enter a confirmed digit, notes for that cell are cleared automatically.