Lesson:Single Candidate Cells
Sometimes, instead of looking at a number, you look at a specific cell. This is called finding a 'Naked Single'.
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Single Candidate Cells — more formally known as Naked Singles — are the simplest and most direct solving technique in Sudoku. A single candidate cell is an empty cell that has only one digit that can legally go there, after all row, column, and box constraints have been applied. You simply look at a cell, eliminate every digit that is already present in its row, column, and box, and if only one digit remains, you place it.
Unlike scanning (which focuses on finding where a digit goes in a box), single candidate analysis focuses on finding which digit goes in a specific cell. Both approaches are complementary, and together they form the complete set of beginner-level techniques. The interplay between them allows solvers to fill most Easy puzzles without any pencil marks.
As you practice, identifying single candidate cells becomes almost instinctive. Your eyes naturally gravitate to nearly-full rows, columns, and boxes — areas where the cell has few remaining candidates and a single candidate cell is likely hiding.
How It Works — Step by Step
Step 1 — Find a cell with many filled neighbors
Look for rows, columns, or boxes that are almost complete (7 or 8 digits filled). The empty cells in near-complete units are the most likely to be single candidates.
Step 2 — List forbidden digits for the cell
For your chosen empty cell, note every digit already present in: (a) its row, (b) its column, and (c) its 3×3 box. Union these three sets — all those digits are forbidden for this cell.
Step 3 — Check if only one digit remains
If exactly one digit from 1–9 is not in the forbidden set, that is the single candidate — place it immediately. If two or more digits remain, this is not a single candidate cell yet.
Step 4 — Update and re-check
Each new placement removes a digit from the candidates of other cells in the same row, column, and box. Always re-check those cells after a placement — a previously two-candidate cell may now be a single candidate.
When to Use This Technique
Continuously — after every placement, check the cells in the same row, column, and box. Single candidate cells are your most reliable and fast technique. Always exhaust them before moving to scanning or more advanced methods.
Worked Examples
A cell in Row 7, Column 4: Row 7 already has 1, 3, 5, 6, 7, 8, 9. Column 4 already has 2, 4. The box containing the cell already has 2, 3, 4, 6. Combined forbidden set: 1, 2, 3, 4, 5, 6, 7, 8, 9. Wait — 2 was not in row 7 but is in column 4 and box. Re-checking: row has 1,3,5,6,7,8,9. Column has 2,4. Box has 2,3,4,6. Union: 1,2,3,4,5,6,7,8,9 — all nine digits! This cell should already be filled; if it is empty, check for an error in your work.
Frequently Asked Questions
Do I need pencil marks to find single candidate cells?
Not always for obvious cases, but pencil marks help for less obvious ones. In near-complete rows and columns, you can often see the single candidate by eye. For cells in less constrained areas, tracking candidates with pencil marks makes the identification much faster.
What comes after exhausting all single candidate cells?
Move to scanning (cross-hatching) for each digit, then to Hidden Singles, and then to more advanced techniques if needed.
Ready to Practice?
Apply this technique on a real puzzle from our daily or practice modes.