For readers playing Sudoku on Cool Math Games, this independent guide focuses on sudoku singles. The examples teach stated local constraints, rather than presenting complete puzzles with proved unique solutions.
A cell with one candidate and a digit with one possible location are related ideas, but they are not the same observation. Naming the distinction makes a deduction easier to check.
A practical starting point
A naked single comes from the candidate set of one cell. A hidden single comes from the locations available to one digit within a unit. In both cases, the conclusion follows from the constraints you have actually established.
Ask whether you are locating a digit or filling a cell
For a naked single, the candidate list for one cell contains just one digit. For a hidden single, one digit has just one possible location within a unit, even if that cell has additional candidates before the deduction. Both rely on complete, accurate information. If notes are partial, the absence of a written candidate elsewhere is not proof that the candidate is impossible there. Read the supplied conditions carefully before applying either label.
A mistake to watch for
Do not assume that a missing digit must go into the first empty cell you notice. First compare the available positions. A candidate is a possibility until the other possibilities have been ruled out.
Worked examples and practice decisions
Try each question before reading its explanation. These examples are deliberately limited to the information they state. They separate a rule or calculation from a broader judgement about a complete game.
1. Candidate exercise 1: identify the forced digit
A row already contains [2, 4, 5, 6, 7, 8] and is missing [1, 3, 9]. For the target empty cell, its crossing column or box excludes [1, 9]. These exclusions are explicitly supplied for this local exercise. Write the missing-digit set before selecting a final number. The order of the numbers in a set is not important; whether a candidate belongs to the set is what matters.
Intersecting the row’s missing digits with the crossing constraints leaves only 3. The candidate set is therefore {3}. This is a naked single for the target cell, provided the stated exclusions are correct. Check the row, column and box again after the placement. The local deduction does not establish the uniqueness of a complete puzzle, and the example does not claim to display a full Sudoku grid.
Countercheck: the target digit 3 is one member of the stated missing set [1, 3, 9]. Choose another member and identify why it cannot replace the target under the supplied conditions. If you remove one of those conditions, reconsider whether the placement is still forced. This tests dependence on the evidence rather than on the appearance of the answer.
2. Candidate exercise 2: identify the forced digit
A unit is missing the digits [2, 6, 7, 9]. Among its empty cells, only cell R can contain 7; the other empty cells have candidate sets that exclude it. Write the missing-digit set before selecting a final number. The order of the numbers in a set is not important; whether a candidate belongs to the set is what matters.
The digit 7 must be placed in cell R within this unit. That is a hidden-single deduction: it depends on one digit having one possible location, even if cell R has additional candidates in its own list. Check the row, column and box again after the placement. The local deduction does not establish the uniqueness of a complete puzzle, and the example does not claim to display a full Sudoku grid.
Countercheck: the target digit 7 is one member of the stated missing set [2, 6, 7, 9]. Choose another member and identify why it cannot replace the target under the supplied conditions. If you remove one of those conditions, reconsider whether the placement is still forced. This tests dependence on the evidence rather than on the appearance of the answer.
3. Candidate exercise 3: identify the forced digit
A row already contains [1, 3, 4, 5, 7, 9] and is missing [2, 6, 8]. For the target empty cell, its crossing column or box excludes [6, 8]. These exclusions are explicitly supplied for this local exercise. Write the missing-digit set before selecting a final number. The order of the numbers in a set is not important; whether a candidate belongs to the set is what matters.
Intersecting the row’s missing digits with the crossing constraints leaves only 2. The candidate set is therefore {2}. This is a naked single for the target cell, provided the stated exclusions are correct. Check the row, column and box again after the placement. The local deduction does not establish the uniqueness of a complete puzzle, and the example does not claim to display a full Sudoku grid.
Countercheck: the target digit 2 is one member of the stated missing set [2, 6, 8]. Choose another member and identify why it cannot replace the target under the supplied conditions. If you remove one of those conditions, reconsider whether the placement is still forced. This tests dependence on the evidence rather than on the appearance of the answer.
4. Candidate exercise 4: identify the forced digit
A unit is missing the digits [4, 6, 7, 8]. Among its empty cells, only cell R can contain 4; the other empty cells have candidate sets that exclude it. Write the missing-digit set before selecting a final number. The order of the numbers in a set is not important; whether a candidate belongs to the set is what matters.
The digit 4 must be placed in cell R within this unit. That is a hidden-single deduction: it depends on one digit having one possible location, even if cell R has additional candidates in its own list. Check the row, column and box again after the placement. The local deduction does not establish the uniqueness of a complete puzzle, and the example does not claim to display a full Sudoku grid.
Countercheck: the target digit 4 is one member of the stated missing set [4, 6, 7, 8]. Choose another member and identify why it cannot replace the target under the supplied conditions. If you remove one of those conditions, reconsider whether the placement is still forced. This tests dependence on the evidence rather than on the appearance of the answer.
5. Candidate exercise 5: identify the forced digit
A row already contains [2, 3, 4, 5, 7, 9] and is missing [1, 6, 8]. For the target empty cell, its crossing column or box excludes [1, 6]. These exclusions are explicitly supplied for this local exercise. Write the missing-digit set before selecting a final number. The order of the numbers in a set is not important; whether a candidate belongs to the set is what matters.
Intersecting the row’s missing digits with the crossing constraints leaves only 8. The candidate set is therefore {8}. This is a naked single for the target cell, provided the stated exclusions are correct. Check the row, column and box again after the placement. The local deduction does not establish the uniqueness of a complete puzzle, and the example does not claim to display a full Sudoku grid.
Countercheck: the target digit 8 is one member of the stated missing set [1, 6, 8]. Choose another member and identify why it cannot replace the target under the supplied conditions. If you remove one of those conditions, reconsider whether the placement is still forced. This tests dependence on the evidence rather than on the appearance of the answer.
6. Candidate exercise 6: identify the forced digit
A unit is missing the digits [4, 6, 8, 9]. Among its empty cells, only cell R can contain 8; the other empty cells have candidate sets that exclude it. Write the missing-digit set before selecting a final number. The order of the numbers in a set is not important; whether a candidate belongs to the set is what matters.
The digit 8 must be placed in cell R within this unit. That is a hidden-single deduction: it depends on one digit having one possible location, even if cell R has additional candidates in its own list. Check the row, column and box again after the placement. The local deduction does not establish the uniqueness of a complete puzzle, and the example does not claim to display a full Sudoku grid.
Countercheck: the target digit 8 is one member of the stated missing set [4, 6, 8, 9]. Choose another member and identify why it cannot replace the target under the supplied conditions. If you remove one of those conditions, reconsider whether the placement is still forced. This tests dependence on the evidence rather than on the appearance of the answer.
7. Candidate exercise 7: identify the forced digit
A row already contains [2, 3, 4, 7, 8, 9] and is missing [1, 5, 6]. For the target empty cell, its crossing column or box excludes [1, 6]. These exclusions are explicitly supplied for this local exercise. Write the missing-digit set before selecting a final number. The order of the numbers in a set is not important; whether a candidate belongs to the set is what matters.
Intersecting the row’s missing digits with the crossing constraints leaves only 5. The candidate set is therefore {5}. This is a naked single for the target cell, provided the stated exclusions are correct. Check the row, column and box again after the placement. The local deduction does not establish the uniqueness of a complete puzzle, and the example does not claim to display a full Sudoku grid.
Countercheck: the target digit 5 is one member of the stated missing set [1, 5, 6]. Choose another member and identify why it cannot replace the target under the supplied conditions. If you remove one of those conditions, reconsider whether the placement is still forced. This tests dependence on the evidence rather than on the appearance of the answer.
8. Candidate exercise 8: identify the forced digit
A unit is missing the digits [3, 5, 6, 8]. Among its empty cells, only cell R can contain 3; the other empty cells have candidate sets that exclude it. Write the missing-digit set before selecting a final number. The order of the numbers in a set is not important; whether a candidate belongs to the set is what matters.
The digit 3 must be placed in cell R within this unit. That is a hidden-single deduction: it depends on one digit having one possible location, even if cell R has additional candidates in its own list. Check the row, column and box again after the placement. The local deduction does not establish the uniqueness of a complete puzzle, and the example does not claim to display a full Sudoku grid.
Countercheck: the target digit 3 is one member of the stated missing set [3, 5, 6, 8]. Choose another member and identify why it cannot replace the target under the supplied conditions. If you remove one of those conditions, reconsider whether the placement is still forced. This tests dependence on the evidence rather than on the appearance of the answer.
Use the guide in your next session
For each exercise, say whether you are narrowing a cell or locating a digit. That one sentence helps prevent an unsupported placement and makes a correction easier if an earlier note was wrong.
Begin with one observation connected to sudoku singles. After checking an example, close the answer and reconstruct the reasoning in your own words. If a step is uncertain, return to that step rather than repeating a result you cannot explain. A small, specific correction provides a clearer next aim than trying to improve every part of a game at once.
Frequently asked questions
Are these complete Sudoku puzzles?
No. They are local constraint exercises with explicitly supplied exclusions. They do not establish the uniqueness of a complete grid.
Can a partial note prove a hidden single?
Not merely by omitting a digit elsewhere. A hidden-single claim needs the other locations to have been ruled out, not simply left unrecorded.
What should I check before entering a digit?
Check the cell’s row, column and box, and identify the constraint that removes every alternative candidate.
More cool math games guides
- Sudoku Beginner Guide: Rules, Units and First Deductions
- Sudoku Candidate Notes: Organise Possibilities Clearly
- Open the relevant game or guide page
Sources and scope
This is an independent, AI-assisted editorial guide. The worked examples and diagrams are original teaching material, not official game screenshots or a claim of personal live-game testing. Game interfaces can change; check the current instructions when controls or rules differ.
