Sudoku techniques, from first puzzle to expert

Every Sudoku with a unique solution can be solved by logic. These are the techniques, in the order you will need them.

1. Hidden singles

Pick a digit, say 5, and look at one box. If the 5 can only go in one cell of that box because the other cells are blocked by 5s in their rows or columns, that cell must be 5. Do the same for rows and columns. Hidden singles solve most easy puzzles on their own, and they are the fastest technique to scan for.

2. Naked singles

Look at a single cell and list which digits it can still take. If only one digit is left after removing everything in its row, column and box, write it in. In the app, turn on Notes and pencil in candidates to make naked singles easy to spot.

3. Naked pairs and triples

If two cells in the same row, column or box both have exactly the same two candidates, for example {3, 8}, those two digits must go in those two cells. You can remove 3 and 8 from every other cell in that unit. The same works for three cells sharing three candidates (a naked triple), even if not every cell shows all three.

4. Hidden pairs

The reverse idea: if two digits can only appear in the same two cells of a unit, those cells contain those digits, and all their other candidates can be erased.

5. Pointing pairs (box to line)

Inside a box, if every possible cell for a digit lies in one row, that digit must be in that row within this box. So you can remove it from the rest of that row outside the box. This is one of the most useful medium-level techniques.

6. Box-line reduction (line to box)

If, within a row, a digit can only appear in cells that all belong to the same box, the digit is locked into that row segment, and you can remove it from the other cells of that box.

7. X-Wing

Find a digit that has exactly two possible cells in each of two different rows, and those cells line up in the same two columns. The digit must occupy two opposite corners of that rectangle, so you can remove it from every other cell in those two columns. Swap rows and columns for the same pattern vertically. Expert puzzles often need an X-Wing or its bigger cousin, the Swordfish (three rows and three columns).

A practical solving routine

1. Scan digits 1 to 9 for hidden singles in every box.
2. When that dries up, fill in candidates for the whole grid.
3. Look for naked singles, then pairs and pointing pairs.
4. After every placement, update candidates in the same row, column and box (the app does this for you).
5. Only reach for X-Wing when nothing else moves.

Common mistakes

• Guessing. If you guess, you may fill the grid with a wrong number you only notice ten moves later.
• Forgetting to update notes after placing a digit.
• Looking only at rows and columns, and missing boxes.

Practise with today’s daily Sudoku, then move up to Hard and Expert.