Pencil mark
A small handwritten or app-rendered note inside a cell indicating which digits the cell could still legally hold. The visible representation of a cell's candidate set.
Pencil marks are the small notations players use to track a cell's remaining candidates. On paper, they're literally marks made with a pencil so they can be erased as candidates are eliminated. In a digital app they're a smaller secondary digit set rendered inside an empty cell, typically arranged in a 3×3 grid that mirrors the layout of the digits 1–9.
When you'll use them
Pencil marks become essential the moment a puzzle stops solving on naked singles alone. On easy puzzles you can hold the candidates in your head; on medium and harder, the candidate sets are too dense to track mentally and pencil marks are how you keep your place. Most digital Sudoku apps offer auto-candidates — the app fills in the marks for you — as a toggleable assist. Sudoku Mountain has it on Classic, off by default, persists per puzzle.
The relationship to candidates
A cell's pencil marks should always equal its candidate set — every digit that's still legal, nothing more, nothing less. Pencil marks are the visible UI for an invisible mathematical fact. Stale pencil marks (digits that should have been erased after a placement elsewhere) are the most common source of wrong moves on harder puzzles. If a technique you applied seems to give a contradiction, your pencil marks are the first place to look: it's almost always a missed elimination, not a broken puzzle.
For a longer take on tidy pencil-marking, see Pencil marks without the clutter; for the visual-pattern angle on reading them, see Reading pencil marks like a shape.
See also
- Candidate— A digit (1–9) a cell could still legally hold — one not yet ruled out by anything in its row, column, or 3×3 box. Every empty cell has between one and nine.
Read more
- Pencil marks without the clutter
When pencil marks help your Sudoku, when they hurt, and the minimal candidate-marking that gets you unstuck without burying the grid.
- Reading pencil marks like a shape
Pencil marks aren't a list of candidates. They're a pattern, and learning to read them as one is the perceptual habit behind every mid-level technique.