Notes in Sudoku
Notes, also known as pencil marks or candidates in sudoku, are small digits you write into empty cells to track which numbers could possibly fit there. This technique transforms a complex, crowded sudoku puzzle into a clear map of possibilities, making hidden logical patterns visible to the solver.
Why use notes?
On easy sudoku puzzles, you can often place digits using only the Last Free Cell and Last Remaining Cell techniques. But on medium, hard and expert levels, the grid is sparse and no single move is obvious. This is where sudoku notes become essential. By listing every valid candidate for every empty cell, you create a complete map of the puzzle's constraints. Patterns like hidden singles, naked pairs and triplets, pointing candidates and claiming strategies all depend on having accurate notes in place. Without notes, these patterns remain invisible.
How to fill notes
For each empty cell on the sudoku grid, consider the digits 1 through 9. Exclude any digit that already appears in the same row, column or 3×3 box as the cell. The remaining digits are your candidates — write them small within the cell. On Sudoked.com, you can activate the Pencil mode and tap to place notes manually, or use long-tap to auto-fill every cell with its correct candidates instantly. Consistent, accurate notes are more important than speed: a single missed candidate can hide a crucial deduction for dozens of moves.
From notes to solutions
Once your sudoku board is filled with accurate notes, a new world of solving strategies opens up. Look for naked singles — cells with only one candidate. Search for naked pairs — two cells in one house that share the same two candidates, allowing you to remove those digits from every other cell in that house. More advanced techniques like hidden pairs, x-wings, swordfish and y-wings all depend on the candidate map that notes provide. The ability to maintain clean, complete notes is what separates casual sudoku players from efficient solvers who can handle expert-level puzzles.