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Sudoku technique · Foundation

Hidden Single

A Hidden Single appears when a digit has only one legal position in a row, column, or box, even if that cell has other candidates.

Topic role: Placement proof
Engine rank: Rank 2

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Learning order

Topic relationships

These links express the same learning and proof relationships used throughout the library.

Teaching aliasLast Remaining Cell
Uses candidate language fromCandidate Notes

Verified example

Work through the example

Hidden Single · Verified example
R1C1: empty	R1C2: empty	R1C3: empty	R1C4: given 4	R1C5: empty	R1C6: given 6	R1C7: target candidates; Before 3, 5, 7	R1C8: empty	R1C9: given 9
R2C1: given 4	R2C2: empty	R2C3: given 6	R2C4: given 7	R2C5: empty	R2C6: empty	R2C7: given 1	R2C8: given 2	R2C9: empty
R3C1: given 7	R3C2: given 8	R3C3: empty	R3C4: given 1	R3C5: empty	R3C6: empty	R3C7: empty	R3C8: empty	R3C9: empty
R4C1: empty	R4C2: empty	R4C3: empty	R4C4: empty	R4C5: empty	R4C6: given 7	R4C7: given 8	R4C8: empty	R4C9: given 1
R5C1: empty	R5C2: empty	R5C3: given 7	R5C4: given 8	R5C5: given 9	R5C6: empty	R5C7: given 2	R5C8: given 3	R5C9: empty
R6C1: given 8	R6C2: empty	R6C3: empty	R6C4: empty	R6C5: given 3	R6C6: empty	R6C7: empty	R6C8: empty	R6C9: given 7
R7C1: empty	R7C2: empty	R7C3: empty	R7C4: given 6	R7C5: given 7	R7C6: empty	R7C7: given 9	R7C8: given 1	R7C9: given 2
R8C1: empty	R8C2: given 7	R8C3: given 8	R8C4: empty	R8C5: empty	R8C6: empty	R8C7: empty	R8C8: empty	R8C9: given 5
R9C1: given 9	R9C2: empty	R9C3: empty	R9C4: empty	R9C5: empty	R9C6: empty	R9C7: empty	R9C8: given 7	R9C9: empty

R1C7 has derived candidates 3, 5, 7. Place 7.

Target cell: R1C7
Derived candidates: 3, 5, 7
Verified result: Place 7
Reviewed unit: Row 1

Choose one row, column, or box and one digit missing from it.
Inspect every empty cell in that unit.
Rule out cells where the digit conflicts with their other units.
If only one legal position remains, place the digit there.

Fixed practice

Track one digit at a time and cross out every position blocked by its row, column, or box.

Fixed practice · R2C3

Hidden Single · Fixed practice
R1C1: given 1	R1C2: empty	R1C3: empty	R1C4: empty	R1C5: given 5	R1C6: given 6	R1C7: empty	R1C8: empty	R1C9: empty
R2C1: empty	R2C2: given 5	R2C3: target candidates; Before 2, 3, 4, 6	R2C4: given 7	R2C5: given 8	R2C6: empty	R2C7: empty	R2C8: empty	R2C9: empty
R3C1: empty	R3C2: given 8	R3C3: given 9	R3C4: empty	R3C5: given 2	R3C6: empty	R3C7: empty	R3C8: empty	R3C9: empty
R4C1: empty	R4C2: given 3	R4C3: empty	R4C4: given 5	R4C5: empty	R4C6: empty	R4C7: empty	R4C8: empty	R4C9: given 1
R5C1: given 5	R5C2: given 6	R5C3: empty	R5C4: empty	R5C5: empty	R5C6: empty	R5C7: empty	R5C8: given 3	R5C9: given 4
R6C1: given 8	R6C2: empty	R6C3: given 1	R6C4: empty	R6C5: empty	R6C6: given 4	R6C7: given 5	R6C8: empty	R6C9: empty
R7C1: given 3	R7C2: empty	R7C3: empty	R7C4: empty	R7C5: empty	R7C6: given 8	R7C7: empty	R7C8: empty	R7C9: given 2
R8C1: given 6	R8C2: given 7	R8C3: empty	R8C4: given 9	R8C5: empty	R8C6: empty	R8C7: empty	R8C8: given 4	R8C9: empty
R9C1: empty	R9C2: given 1	R9C3: empty	R9C4: empty	R9C5: empty	R9C6: given 5	R9C7: empty	R9C8: empty	R9C9: empty

R2C3 has derived candidates 2, 3, 4, 6. Place 6.

Target cell: R2C3
Derived candidates: 2, 3, 4, 6
Verified result: Place 6
Reviewed unit: Column 3

Common mistakes

Common mistakes

Looking only for cells with one candidate, which identifies a Naked Single instead.

Declaring an only position before checking every empty cell in the selected unit.

Review information

What is verified here

The verifier tests the chosen digit in every empty cell of the selected unit and accepts the move only when exactly one legal position exists.

Fixture ID: p2-hidden-single-row-01 / p2-hidden-single-column-01
Engine version: legen-sudoku-engine/1.3.0
Verification status: Independent placement verifier passed
Facts reviewed: 2026-07-18

Technique terminology and examples are tied to the published engine version. See the review information below for scope and dates.