Sudoku-YFoundations
Naked Single vs Hidden Single: cell or digit?
Both techniques can establish an answer without guessing. One examines a cell; the other examines a digit within a row, column or box.
Practise on a board ↓Interactive practice · Your saved game is unchanged
Find the missing digit
Row A is missing one digit. Choose the value for A5.
An original one-row Naked Single exercise. Large digits are placed values. A row needs 1–9 exactly once. This is not a complete puzzle.
Progress: 0 / 1
Show reasoning and answer
Row A already has 1, 2, 3, 4, 6, 7, 8 and 9. Only 5 remains for A5.
LOOK · TRY · UNDERSTAND
Try it and see why
One cell, or one digit?
Row A needs 2, 5, 7, with only A1, A4 and A8 empty. The notes shown are complete.
Our own partial teaching board, not a full puzzle. Large digits are placed values; small digits are candidates. Dots mean the cell is not described here. Complete candidate lists are part of the stated scenario, not calculated from the dots.
Green outline = evidence · Purple “try” = an assumption · Crossed out = removed candidate
Which explanation gives a justified placement?
See the reasoning
A8 has only 5, so it is a Naked Single. A1 and A4 still need more thought.
Digit set 1/9 · A different digit set in the same pattern, not a new full puzzle.
Read the source principles: HoDoKu — Singles
Check the condition, not just the shape
In a separate example, row A needs 2, 5 and 7. Its only empty cells are A1, A4 and A8, with the complete candidates shown. Can we place A1 = 7 now?
Check the reasoning
Yes. A1 has two candidates, but 7 has no other position in the row. If A1 were 2, A4/A8 would need to take 5 and 7, although neither allows 7. That is a contradiction. If A4 also allowed 7, this conclusion would no longer follow.
On your game: follow one digit through every empty cell in a single unit. Exactly one position must allow it. Incomplete handwritten notes are not evidence of a unique position.
Naked Single: inspect one cell
If sound eliminations leave a cell with just one candidate, place it. For example, C4={8} establishes C4=8. You do not need to resolve the other occurrences of 8 first.
Hidden Single: inspect one digit
If a unit still needs 7 and only one cell can contain it, that cell must be 7 even if it has other candidates. Scanning for a digit with only one available position is a useful way to inspect rows, columns and boxes.
Compare the two ideas
Suppose a row has empty cells A1={2,7}, A4={2,5}, A8={2,5}, and its other cells are filled. The row still needs 7. Only A1 allows it, so this is a Hidden Single.
Try before revealing
Why can A1 be 7 while its notes still say {2,7}?
Show explanation
The row needs 7 and every other position in that row is ruled out. You must have checked the whole row, not just a convenient selection of cells.
Check your notes first
Incomplete notes can create a false appearance of a unique position. Check against the placed digits before committing. Sudoku-Y hints illustrate singles on your current board; you can follow the reasoning and enter the answer yourself.
References and further reading
Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.