Buddy cells are cells that share a house: the same row, column, or box. Look at R5C8 in this puzzle; its buddy cells are highlighted in grey.
To begin with, R5C8 could be any digit from 1 to 9, so we note all nine as candidates. Notes like these are called pencil marks.
Row 5 already contains 2, 9, and 3. A digit can appear only once per row, so R5C8 cannot be any of them.
Eliminate 2, 3, and 9 from the candidate list.
The box of R5C8 already uses 1, 5, 6, and 7, so those go too.
Finally, the column contains 2 and 4. The 2 is already gone, so we remove the 4.
Only one possibility is left: 8. So R5C8 must be 8. That is a naked single.
Here is another naked single. Can you see the answer for R4C6?
Its buddy cells contain every digit from 1 to 9 except 8, so R4C6 must be 8.
Lesson complete
Nice work - you went through every step. Ready for the next lesson?
Next: Locked Candidates in SudokuWhat a naked single is
A naked single is a cell whose candidate list has shrunk to exactly one digit. Every other digit from 1 to 9 already appears in one of the cell's buddy cells, so the last remaining candidate is the answer.
It is called "naked" because the answer sits alone and exposed in the cell itself, unlike a hidden single where you must check the whole house to see it.
Worked example: narrowing one cell to a single candidate
The cleanest way to understand naked singles is to build the candidate list for one cell and cross digits off house by house.
A second example
With practice you can spot naked singles without writing every pencil mark, especially in cells surrounded by many solved buddies.
Naked single vs hidden single
A naked single is about one cell: the cell itself has only one candidate left. A hidden single is about one house: a digit has only one legal cell in a row, column, or box, even though that cell may still show several candidates.
Both moves are 100% safe placements. Scan for them constantly; most easy and medium puzzles can be solved with singles alone.
Common mistakes
Do not place a digit just because it fits the row. A naked single requires checking all three houses: row, column, and box.
If you use pencil marks, keep them updated. A stale candidate list can make a cell look like a naked single when it is not.
Frequently asked questions
Do I need pencil marks to find naked singles?
Not always. Cells with many solved buddy cells can be checked mentally. Pencil marks make naked singles obvious in harder puzzles.
Are naked singles enough to solve a puzzle?
Easy puzzles usually fall to Full House, hidden singles, and naked singles. Harder puzzles add candidate eliminations before new singles appear.
What should I learn after naked singles?
Learn to keep clean pencil marks, then move on to locked candidates, the first real elimination technique.
Practice next
Naked singles appear at every difficulty: easy puzzles are full of them, and even evil puzzles come back to them after each elimination. Solve a few puzzles focusing only on spotting them quickly.