Sudoku techniques
The extra techniques used by Medium and Hard
Start with singles. When they stop, these candidate patterns can remove possibilities without guessing.
Medium
Locked candidates
If every possible position for one digit inside a 3 × 3 box lies in the same row or column, remove that digit from the rest of that row or column. The reverse pattern—one row or column confined to a single box—works too.
Naked and hidden pairs
A naked pair is two cells in one unit whose candidates are the same two digits; those digits can be removed from the other cells in the unit. A hidden pair is two digits that can occur only in the same two cells; other candidates can be removed from those cells.
Hard
Naked triples
In one row, column, or box, choose three unsolved cells whose combined candidates contain exactly three digits. Those three digits must occupy the chosen cells, so remove them from every other unsolved cell in that unit.
Hidden triples
In one row, column, or box, choose three digits whose combined possible positions are confined to exactly three cells. Keep those three digits in the chosen cells and remove every other candidate from those cells.
Row and column X-Wings
For one digit, find two rows where that digit has exactly two candidate positions in each row, aligned in the same two columns. The digit must occupy opposite corners of that rectangle, so remove it from those two columns outside the selected rows. The column-based form is symmetric: use two columns with exactly two aligned candidate rows, then remove the digit from those rows outside the selected columns.
How website hints work
A hint recalculates candidates from the current board, ignores pencil notes, explains the logical steps, and offers one placement. Applying it changes one cell and can be undone. If the board changes first, the hint is discarded and must be recalculated.