🔍 Hidden Single

Hidden Single is the natural partner to the Naked Single. While a Naked Single focuses on a cell that has only one candidate, a Hidden Single focuses on a digit that can only go in one cell within a unit. The digit is "hidden" because the cell it belongs to may appear to have several possible candidates at first glance — you only discover the truth by scanning an entire row, column, or box for that specific digit.

What is a Hidden Single?

A Hidden Single exists when a digit appears as a legal candidate in exactly one cell within a row, column, or 3×3 box. No matter what other candidates that cell has, this digit must go there — because there is nowhere else in the unit for it to be placed. The digit is "hidden" in the sense that the cell's other candidates distract you from the obvious fact that this particular digit has no alternative home in its unit.

Step-by-step guide

1. Choose a digit to scan. Pick any digit 1–9. Good candidates to start with are digits that appear many times on the board — fewer empty cells means fewer positions to check.
2. Choose a unit to scan. Pick a row, column, or 3×3 box and focus on its empty cells.
3. Eliminate illegal positions. For each empty cell in the unit, ask: can this digit legally go here? A digit cannot go in a cell if it already appears in the same row, column, or box as that cell. Cross those positions off mentally.
4. If only one cell remains, place the digit there. That is the Hidden Single.
5. Repeat for all digits and all units. Systematically scanning each digit across all rows, columns, and boxes will reveal every Hidden Single on the board.

Digit 7 can only go in the amber cell — all other empty cells in this box are blocked by row or column

Why does it work?

Every digit must appear exactly once in each row, column, and 3×3 box. If a digit can only be placed in one cell within a unit — because all other empty cells in that unit are blocked by row or column constraints — then it must go in that one cell. There is no alternative. Even though the destination cell may have other candidates, those candidates do not affect this reasoning: we are not asking "what goes in this cell?" but "where can digit X go in this unit?"

This shift in perspective — from cell-first to digit-first thinking — is what distinguishes Hidden Singles from Naked Singles and unlocks a new layer of deductions.

Three types of Hidden Single

• Row Hidden Single. A digit has only one valid column position within a row. Scan horizontally across the row, crossing off cells where the digit already appears in that column or box.
• Column Hidden Single. A digit has only one valid row position within a column. Scan vertically.
• Box Hidden Single. A digit has only one valid cell within a 3×3 box. This is often the easiest to spot — the box is visually compact and experienced solvers find box Hidden Singles without pencil marks, by simple visual inspection.

Scanning strategy

A systematic approach prevents missed Hidden Singles:

1. For each digit 1–9, count how many times it already appears on the board.
2. If a digit appears 8 times, the 9th placement is immediate — scan the one remaining row, column, and box that does not have it yet.
3. If a digit appears 7 times, only two placements remain. These are usually the easiest Hidden Singles to find.
4. Work through each digit in turn. This is called digit scanning or cross-hatching and is the fastest way to clear Hidden Singles from a board.

Naked vs Hidden — comparison

Using pencil mode in SudokuStreak

Activate pencil mode and fill in all candidates for every empty cell. Once all candidates are recorded:

• Any cell with exactly one candidate is a Naked Single — place it immediately.
• For each unit (row, column, box), scan for any digit that appears as a candidate in only one cell — that is a Hidden Single. Place that digit and remove it as a candidate from all other cells in that unit.

Always exhaust all Naked Singles and Hidden Singles before trying harder techniques. These two alone solve the vast majority of Easy and Medium puzzles, and a significant portion of Hard ones.

Common mistakes

• Not scanning all three unit types. A digit might not have a Hidden Single in any row but might have one in a box. Scan rows, columns, and boxes separately.
• Confusing "can go here" with "must go here". A Hidden Single requires exactly one valid cell in the unit, not two or more.
• Forgetting to check after placements. Every placement removes a candidate from up to 20 peer cells, potentially creating new Hidden Singles. Always re-scan after placing a digit.
• Skipping to advanced techniques too soon. Hidden Singles often hide in plain sight. Experienced solvers can miss them when rushing. Always complete a full digit scan before resorting to Naked Pairs or X-Wing.