Learn how to use candidate numbers (Notes / Pencil Mode) in Sudoku with visual examples. Discover how tracking possibilities reveals Naked Singles and Pairs.
Using the "✎ Notes (Memo)" button on our game board allows you to jot down small numbers from 1 to 9 inside any empty cell. These small numbers are called Candidate Numbers (or Pencil Marks), and mastering them is the single biggest leap from beginner to intermediate solver.
Beginners usually ask, "Where in this box does digit 5 go?"
With candidate notes, you can ask the reverse question for a single intersection cell: "Which digits from 1 to 9 are NOT yet present in this cell's row, column, or 3×3 box?"
| . | . | . | . | 1 | . | . | . | . |
| . | . | . | 2 | 3 | . | . | . | . |
| . | . | . | . | . | . | . | . | . |
| . | . | . | 4 | 5 | . | . | . | . |
| 6 | 7 | . | . | 9! | . | . | 8 | . |
Look at the center cell (Row 5, Column 5): its row already contains 6, 7, 8; its column contains 1, 3, 5; and its 3×3 center box contains 4 (plus 2 in the top box if scanned). When 8 different digits see a single cell, only 1 candidate remains!
