Sudoku FAQ
Common questions about how to start a Sudoku puzzle, the difference between naked singles and hidden singles, and when to use pencil marks.
How do I start a Sudoku puzzle?
Scan rows, columns, and 3x3 boxes for numbers that appear frequently. The more numbers already placed in a row, column, or box, the easier it is to deduce the missing one. Look for near-complete units first.
What is a naked single versus a hidden single?
A naked single is a cell where only one number fits among all candidates. A hidden single is a number that can only go in one cell within a row, column, or box even though the cell may have other possibilities.
When should I use pencil marks?
Use pencil marks on hard puzzles when you need to track multiple possibilities for a cell. On easy and medium puzzles, careful scanning usually avoids the need for them. On hard puzzles, pencil marks reveal hidden singles and naked pairs you cannot see otherwise.