A candidate is a digit that has not yet been ruled out for an unsolved cell.
To find a cell's candidates, start with 1–9 and remove every digit already blocked by its row, column, or box. What remains is the current set of legal possibilities.
Candidates matter because Sudoku stops being only about “which number goes here?” once the puzzle gets harder. Intermediate and advanced techniques often ask instead:
Which possibilities can I eliminate, reserve, or connect?
How do you find the candidates for a cell?
Take an empty cell and check its three units.
Suppose the digits already present in its row, column, and box rule out:
1, 2, 3, 5, 6, 8, 9
The only digits not ruled out are:
4, 7
So the candidate set for that cell is:
{4, 7}
That does not mean you should choose between 4 and 7. It means both remain possible until more logic removes one of them or proves one must be placed.
Candidates are possibilities, not guesses
This distinction matters.
A candidate is supported by the current rules: nothing in the row, column, or box rules it out yet.
A guess is a speculative decision to place one candidate without proving that it must be correct.
Good candidate tracking helps you avoid that jump.
What are pencil marks?
Pencil marks are the visual notes used to record candidates inside cells.
On paper, players literally write small digits in pencil. In a digital Sudoku, the same idea is usually called notes or candidate mode.
For example, a cell displaying:
2 5 8
means you are currently tracking {2, 5, 8} as its candidates.
A dedicated Pencil Marks guide will cover note-taking styles and when full candidate notation is useful.
Why candidates change
Candidates are based on the current grid, so they change whenever new information appears.
If you place 5 in a cell, every unsolved peer of that cell can eliminate 5 from its candidates.
That removal may create a chain of consequences:
- 5 is placed.
- Another cell changes from
{5, 7}to{7}. - That cell becomes a Naked Single and is solved as 7.
- The new 7 removes candidates from its peers.
- Another technique may become available.
This is why candidate maintenance is not just bookkeeping. It is part of the solving process.
Naked Singles from candidates
If an unsolved cell has exactly one candidate, the cell is solved.
Example:
{8}
The cell must contain 8 because every other digit has been eliminated.
This is called a Naked Single.
Hidden Singles from candidates
A Hidden Single works differently.
Imagine a row where candidate 6 appears in only one unsolved cell. That cell might display {2, 6, 9}, so it is not a Naked Single. But if no other cell in the row can contain 6, then 6 must be placed there.
The candidate is “hidden” among other possibilities in the cell, but unique within the unit.
Candidates and subsets
Candidates also create elimination patterns.
If two cells in the same unit contain only {3, 8}, those two digits must occupy those two cells in some order. No other cell in the unit can therefore contain 3 or 8.
That is a Naked Pair.
Triples, quads, hidden subsets, Wings, Fish, and Chains all rely on candidate relationships in more advanced ways.
Do you need to write every candidate?
Not always.
For many Easy puzzles, scanning and simple Singles may be enough. Some solvers prefer to add notes only when the puzzle becomes less obvious.
Other players use full candidate notation early because it makes patterns easier to inspect systematically.
VeyraPlay should teach both approaches rather than pretending one notation habit is mandatory.
How to keep candidates accurate
- Never add a digit that is already blocked by the row, column, or box.
- When a digit is placed, remove that digit from all unsolved peers.
- Re-check candidates after major eliminations.
- Do not keep a candidate simply because it was possible several moves ago.
- If an advanced pattern depends on candidates, validate the candidate state before applying it.
Candidate modes in VeyraPlay
Future product behavior can support several layers without changing the concept:
- manual notes entered by the player
- optional automatic candidate generation
- teaching overlays in Learn
- technique-specific highlights in Practice
The editorial definition stays the same: a candidate is a digit that remains legally possible in an unsolved cell at that point in the puzzle.
FAQ
How many candidates can a Sudoku cell have?
An unsolved cell can have several candidates early in a puzzle. As more digits are solved and candidates are eliminated, the set usually becomes smaller.
Is a cell solved when it has two candidates?
No. Two candidates mean two possibilities remain unless another deduction resolves the cell.
Are notes and candidates the same thing?
Candidates are the logical possibilities. Notes or pencil marks are the way a player or app records them.
Should candidates update automatically?
That is a product preference, not a Sudoku rule. Automatic updates can reduce bookkeeping; manual notes can provide a more traditional solving experience. Learn may use automatic overlays when the lesson is about logic rather than note entry.
What to learn next
Continue with Pencil Marks for notation, then learn Naked Singles and Hidden Singles. Once those are comfortable, candidate relationships lead naturally to Locked Candidates and Naked Pairs.