Pencil marks are the visible notes you use to keep track of candidates inside unsolved cells.
The distinction is useful:
- a candidate is a logical possibility;
- a pencil mark is how you record that possibility.
In digital Sudoku, the feature may simply be called Notes. Whatever the interface calls it, the purpose is the same: keep enough of the candidate state visible to compare cells and make reliable eliminations.
What do pencil marks mean?
If a cell contains small notes such as:
2 5 8
those notes mean that 2, 5, and 8 are currently being considered as candidates for that cell.
A well-maintained candidate set should contain only digits that are not already blocked by the cell's row, column, or box.
That does not mean one of those digits should be guessed. The notes are a record of possibilities, not a choice between them.
Pencil marks and candidates are related, but not identical
The terms are often used interchangeably in Sudoku apps, but there is a useful distinction:
- a candidate is a digit that remains logically possible in a cell;
- a pencil mark is the visible note used to record that candidate.
For example, if a cell can legally contain only 3 or 7, its candidate set is {3, 7}. Writing small 3 and 7 notes in the cell is the pencil-mark notation for that information.
If candidates are new to you, read Sudoku Candidates before using full notes throughout a puzzle.
Do you need pencil marks to solve Sudoku?
No. Many easy puzzles can be solved with visual scanning and straightforward placements without writing every candidate.
As puzzles become harder, however, pencil marks become more useful because many techniques depend on seeing candidate relationships across several cells. Naked Pairs, Hidden Pairs, Locked Candidates, X-Wings, Wings, and Chains are much easier to reason about when the relevant possibilities are visible.
The important point is not to fill the grid with tiny digits as early as possible. The goal is to record enough information to make the next logical step easier to see.
Two common ways to use pencil marks
1. Selective notes
With selective notes, you record only candidates that seem useful.
For example, you might mark where a particular digit can go in one box, or add notes to a small group of cells when the puzzle becomes less obvious.
This keeps the board visually clean and can work well for easier puzzles.
The drawback is that incomplete notes cannot safely support every advanced technique. If a technique assumes you are seeing all remaining candidates in a unit, missing notes may lead you to a false conclusion.
2. Full candidate notation
With full candidate notation, every unsolved cell displays all digits that can still legally fit there.
This creates more visual information, but it also makes systematic techniques easier to identify.
Full notation is especially useful when:
- simple scanning has stopped producing placements;
- you are learning candidate-based techniques;
- you want an app to maintain candidates automatically;
- you are analyzing a difficult puzzle carefully.
Neither style is universally “correct.” VeyraPlay should support both habits and teach when each one is useful.
How to add correct pencil marks
For one empty cell:
- Start with digits 1–9.
- Remove every digit already present in the same row.
- Remove every digit already present in the same column.
- Remove every digit already present in the same box.
- Record the remaining digits as candidates.
How pencil marks change after a move
Pencil marks are not permanent.
Suppose a cell contains candidates {4, 7}. If a 4 is later placed in the same row, 4 is no longer possible in that cell. Its notes should become:
{7}
That immediately creates a Naked Single.
This is one reason accurate candidate maintenance matters. One placement can remove several notes, and those removals can expose a new technique.
Manual notes vs automatic candidates
Digital Sudoku can manage notes in different ways.
Manual notes
You add and remove every note yourself.
This is closest to solving on paper and forces you to maintain the candidate state accurately.
Automatic candidates
The game calculates all currently legal candidates for you.
This removes bookkeeping and makes pattern recognition easier, but it also means some techniques — especially Naked Singles — become visually much more obvious.
Hybrid notes
A game can calculate legality while still letting you decide which notes to display.
For VeyraPlay, the important product rule is clarity: the interface should distinguish logical candidate state from what the player has chosen to note, rather than silently treating them as the same thing.
Common pencil-mark mistakes
Treating a note as a guess
A candidate means “still possible,” not “probably correct.” Do not place a digit merely because it is one of two notes in a cell.
Leaving impossible candidates behind
If a digit is placed in a row, column, or box, peers can no longer keep that digit as a valid candidate.
Using incomplete notes as if they were complete
A Naked Pair or similar pattern depends on the candidate state being accurate. Selective notes are fine, but you need to know when a deduction assumes full notation.
Adding notes without first checking all three units
A digit can be allowed by the row but blocked by the column or box. Candidate legality always depends on all three.
Filling every note before checking for easy moves
Notes are a tool, not a ritual. A quick scan may reveal direct placements before full candidate notation is necessary.
A practical pencil-mark workflow
A simple workflow for a human solver is:
- Scan for obvious placements first.
- When progress slows, add or reveal candidates.
- Check for Naked Singles and Hidden Singles.
- Update notes after every placement or elimination.
- Move to candidate patterns such as Locked Candidates and subsets only when necessary.
This keeps the board useful rather than merely busy.
FAQ
Are pencil marks allowed in Sudoku?
Yes. Pencil marks are simply notes used by the solver. They do not alter the puzzle or its rules.
Should I write every possible number in every empty cell?
Not necessarily. Full candidate notation is useful for systematic solving, while selective notes can keep easier puzzles cleaner. What matters is knowing whether the technique you are using depends on a complete candidate state.
Why did one of my notes disappear?
If a digit is placed in the same row, column, or box, that digit is no longer a legal candidate for the cell and should be removed.
Are automatic candidates cheating?
They automate bookkeeping, not the logical deduction itself. Whether you use them is a play preference. For learning advanced techniques, automatic candidates can make the relevant pattern easier to study.
What is the difference between notes mode and normal number entry?
Normal entry places a solved digit in a cell. Notes mode records one or more possibilities without claiming the cell is solved.
What to learn next
Once you are comfortable reading candidates, learn Naked Singles and Hidden Singles. After that, candidate notation becomes the foundation for Locked Candidates, Naked Pairs, and other intermediate techniques.