Fill the 9x9 grid so each row, column, and 3x3 box contains digits 1-9 without repetition.
Whether you're new to Sudoku or chasing Evil-difficulty puzzles, these techniques will sharpen your solving skills.
A naked single is a cell where only one digit is possible. Scan every empty cell and check which digits already appear in its row, column, and 3×3 box. If only one digit is missing, fill it in immediately. This alone solves most Easy puzzles.
A hidden single is a digit that can only appear in one cell within a row, column, or box — even though that cell may still have other candidates. Go through each unit and check: is there a digit that fits in exactly one spot? If yes, place it.
For Medium difficulty and above, use Notes mode to mark every possible candidate in each empty cell. This makes advanced patterns visible. Press N or click the Notes button to toggle pencil-mark mode.
If two cells in the same unit both contain exactly the same two candidates, those digits cannot appear anywhere else in that unit. Eliminate them from other cells. Extend this logic to naked triples (three cells, three shared candidates).
If a candidate within a 3×3 box is limited to one row or column of that box, it cannot appear elsewhere in that row or column outside the box. Use this to eliminate candidates from the rest of the row or column.
For Hard and Expert puzzles: if a digit appears in exactly two cells in each of two different rows, and those cells share the same two columns, the digit can be eliminated from all other cells in those columns. The same logic applies to columns.