Pencil Mark
Small notations of candidate digits written in a cell. An essential tool for solving intermediate and advanced Sudoku puzzles.
Pencil marks are small notations of candidate digits written in each cell. On paper Sudoku, they are written small in the corners of cells; in digital apps, they are entered using a notes mode. The term is often used interchangeably with "candidate."
Notation Conventions
- 2, 5 and 9 still remain as candidates
On the left is the key: a single cell divided into nine positions, showing where each digit is written (1 top-left through 9 bottom-right). On the right is a cell marked up with that same convention, where only 2, 5 and 9 are left. Because every digit has a fixed position, an empty position alone tells you the digit has been ruled out, without reading any number at all.
On paper, the common method is to divide the cell into a 3x3 grid and write each digit in its corresponding position: 1 in the top-left, 2 in the top-center, 3 in the top-right, and so on to 9 in the bottom-right. This layout allows instant visual recognition of which digits are present.
Automatic Management in Digital Apps
Digital apps can automatically calculate and remove candidates. When a digit is confirmed, pencil marks in related cells are automatically updated. This eliminates the cost of manual note management, allowing players to focus purely on logical reasoning.