Sudoku

Sudoku techniques

Start with the candidates in a cell, then the positions for a digit. Follow this order and return to singles after each elimination.

Easy: naked and hidden singles. Medium: pairs and locked candidates (pointing / box-line). Hard: triples. Expert: X-Wing / Swordfish / XY-Wing. The highest technique used to solve a puzzle sets its tier; puzzles the ladder cannot solve are outside these four tiers.

Notes and automatic candidates are optional at every stage and do not set the tier.

1. Naked single: the last possible number in a cell

   Use a naked single when one cell has only one digit left that its row, column, and box still allow.

2. Hidden single: scanning a row, column, or box

   Use a hidden single when a digit has only one legal cell left in a box, a row, or a column.

3. Notes: pencil marks for when you are stuck

   Use notes when singles still leave empty cells. Pencil marks are candidates you choose to write, not a difficulty.

4. Naked pairs and hidden pairs: two cells, two digits

   Use a naked or hidden pair when two cells, or two digits, lock each other inside one row, column, or box.

5. Pointing: a digit locked to one line

   Use pointing or box-line reduction when a digit is locked to one row inside a box, or to one box inside a row.

6. X-Wing: two rows and the same two columns

   Use X-Wing when one digit is limited to the same two columns in two different rows.

7. Sudoku techniques when stuck: what to check next

   Sudoku techniques for when you're stuck: try singles, pairs, pointing, triples, then X-Wing, returning to singles after each deletion.

8. Easy, medium, hard and expert Sudoku: what's the difference?

   The difference between easy, medium, hard and expert Sudoku is the highest technique the grading ladder uses, not the number of givens.