Pencil Marks
Tracking Candidates to Unlock Harder Puzzles
By Minimal Sudoku Team•Last updated:
Pencil marks (also called candidates or notes) are small numbers written in cells to track which digits could possibly go there. They're essential for solving medium and harder puzzles.
Quick Summary
- What: Small numbers showing possible candidates for a cell
- Why: Track possibilities, spot patterns, avoid mistakes
- When: When basic scanning isn't finding placements
- Where: Usually in cell corners or center
Why Use Pencil Marks?
Pencil marks serve several purposes:
- Memory aid: Don't recalculate possibilities for every cell repeatedly
- Pattern recognition: Spot Naked Pairs, Hidden Pairs, and other patterns
- Error prevention: Visualize constraints to avoid mistakes
- Enable advanced techniques: Many techniques require seeing all candidates
When to Start Noting
⏰ The Right Time
Start pencil marking when basic scanning stops finding placements. For easy puzzles, you might never need them. For hard puzzles, start earlier.
✅ Good Time to Start
- • Scanning finds nothing
- • You're rechecking the same cells
- • Grid feels "stuck"
❌ Too Early
- • Still finding hidden singles
- • Many obvious placements left
- • Early in an easy puzzle
How to Add Pencil Marks
1
Pick an empty cell
Choose any cell without a placed digit.
2
Check the row
Which digits 1-9 are already in this row? Those are eliminated.
3
Check the column
Eliminate additional digits that appear in the column.
4
Check the box
Eliminate any remaining digits in the 3×3 box.
5
Write remaining candidates
The digits NOT eliminated are your pencil marks for this cell.
Notation Styles
Center Notation
Write all candidates in the center of the cell.
Best for: Full candidate marking
Corner Notation (Snyder)
Write candidates in corners, only when limited to 2 cells in a box.
Best for: Snyder method
IntermediateSnyder Notation
Learn the efficient corner-marking system used by world champions.
Maintaining Your Notes
Keep Notes Updated!
When you place a digit, immediately remove that candidate from all cells in the same row, column, and box. Outdated notes lead to errors.
Good note maintenance is crucial:
- After each placement, scan affected cells and remove the placed digit
- After eliminations from techniques, update notes accordingly
- If a cell has only one candidate left, place it!
Tips for Success
Don't Note Everything
You don't need to pencil mark every cell. Focus on areas where you're stuck or where patterns might exist.
Use Digital Tools
Apps like Minimal Sudoku have built-in candidate modes that auto-update when you place digits. Much easier than paper!
Start with Snyder
For most puzzles, Snyder Notation gives you 80% of the benefit with 20% of the work. Only switch to full notation when needed.