Sudoku Strategies Explained
Every Sudoku can be solved with logic. These are the techniques you need, from the first easy steps to expert patterns, in the order you will meet them.
1. Singles (Easy)
Naked single: a square where only one number is still possible because the other eight already appear in its row, column or box. Hidden single: a number that has only one possible square left in a row, column or box, even if that square could hold other numbers too. Easy puzzles need nothing else.
2. Pointing pairs and naked pairs (Medium)
Pointing pair (locked candidates): if, inside one box, a number can only go in squares that lie in the same row, then that number must be in that row within this box, so you can remove it from the rest of that row outside the box. It works the same for columns.
Naked pair: two squares in the same row, column or box whose notes are the same two numbers, say {4,8}. One will be 4 and the other 8, so remove 4 and 8 from every other square in that unit.
3. Hidden pairs, triples and the X-Wing (Hard)
Hidden pair: two numbers that can only go in the same two squares of a unit. Those squares must hold those two numbers, so you can delete any other notes from them. Triples work the same way with three numbers in three squares.
X-Wing: pick a number. If in two different rows it can only go in the same two columns, those four squares form a rectangle. The number must sit in opposite corners, so it can be removed from every other square in those two columns.
4. XY-Wing, Swordfish and quads (Expert)
XY-Wing: three squares with two notes each, such as a “pivot” {A,B} that sees one square {A,C} and another {B,C}. Whatever the pivot turns out to be, one of the two wings must be C, so any square that sees both wings cannot be C.
Swordfish: the X-Wing idea stretched to three rows and three columns. Quads: pairs and triples extended to four numbers in four squares.
Practice tips
- Always do the simple steps first. Most moves in any puzzle are singles.
- Fill in notes for a whole box or row at a time, and keep them up to date.
- When a technique removes a note, look again for new singles straight away.
Practise each level: Easy for singles, then Medium, then Hard and Expert. Our game grades every puzzle by these techniques, and its hint names the technique you need next.
Frequently asked questions
What is the hardest Sudoku technique?
There are rarer patterns beyond the ones here, but XY-Wing, Swordfish and quads cover the Expert puzzles on this site.
Do I ever need to guess?
No. All our puzzles can be solved by logic alone.
Which technique should I learn after singles?
Pointing pairs and naked pairs. They unlock most Medium puzzles.