Sudoku solving techniques.
Learn naked singles, hidden singles, locked candidates and naked pairs with clear Sudoku explanations, then practice each technique on a free puzzle.
Put it into practice →Only candidate — also called a naked single
A naked single is an empty cell with only one candidate. Check all three of its groups. For example, if its row rules out 1, 2 and 3, its column rules out 4 and 5, and its box rules out 6, 7 and 8, the cell must be 9. The reason is elimination of all eight alternatives.
Only place — also called a hidden single
A hidden single is about a digit within one group. Suppose several cells have multiple candidates, but only one cell in a box can contain 4. That cell must be 4, even if its own candidate list contains other digits. Check each missing digit across all empty cells in the group.
Locked candidates — pointing and claiming
If every candidate 6 in a box lies on the same row, remove 6 from the rest of that row outside the box. This is pointing. The reverse works too: if every candidate 6 in a row falls within one box, remove 6 elsewhere in that box. This is claiming. Both use the fact that each group needs exactly one 6.
Naked pairs — two cells reserve two digits
Find two cells in one row, column or box that each have exactly the same two candidates. If both cells are {3, 8}, one is 3 and the other is 8. You do not yet know which is which, but you can remove 3 and 8 from every other cell in the shared group. A cell with a third candidate does not qualify.
Why a hint can include several steps
An elimination may not place a digit immediately. A locked candidate can remove one option, expose a pair, and then create a single. Our hint panel follows those eliminations until it reaches a placement, and explains the chain. The candidates used by the solver are calculated independently of your handwritten notes.
Practice the technique you are learning
Use easy puzzles to get comfortable with singles. Choose medium when you want locked candidates, and hard for naked pairs. Work slowly enough to explain why a move follows. When you can repeat the reasoning on a new board, you have learned something more useful than one answer.