Guide
Sudoku techniques, from easiest to hardest
Every Sudoku is solved with the same short list of techniques. Learn them in this order, and try them in this order: the first one that works is the one to use.
Ninesmith Sudoku ·
In short
- Singles place a digit: a cell with one option left, or a digit with one place left. Most puzzles are mostly singles.
- Pairs, triples and locked candidates place nothing. They strike candidates from your notes, and the singles they open do the rest.
- X-Wing, Swordfish, Y-Wing and Unique Rectangle are for hard puzzles, once everything above has run out.
Stuck in a puzzle right now? What to do when you're stuck walks through the same list as a checklist.
Singles
- Last free cell
- A row, column or box with eight digits has its ninth decided. The app calls it Obvious Elimination.
- Naked Single
- A cell that sees eight different digits in its row, column and box can only take the ninth.
- Hidden Single
- A digit with only one place left in a row, column or box. The cell may take other digits; the digit has nowhere else to go. This is what scanning finds.
Pairs and triples
- Naked Pair
- Two cells in a house with the same two candidates and nothing else. Those two digits go there, so they come out of the rest of the house.
- Naked Triple
- Three cells in a house that share only three candidates between them. Those three digits come out of the rest of the house.
- Hidden Pair
- Two digits that fit in only the same two cells of a house. Every other candidate comes out of those two cells.
- Hidden Triple
- Three digits that fit in only the same three cells of a house. Every other candidate comes out of those cells.
A house is a row, a column or a box: any group of nine cells that must hold 1 to 9.
Locked candidates
- Locked Candidates
- Inside a box, a digit fits only on one row or column, so the rest of that line loses it. It works the other way too, from a line into a box. Also known as pointing pairs and box/line reduction.
Advanced techniques
- X-Wing
- A digit fits in only the same two columns in two rows (or the same two rows in two columns). It comes out of the rest of those lines.
- Swordfish
- The X-Wing across three rows and three columns.
- Y-Wing
- Three cells with two candidates each, AB, AC and BC, where AB sees the other two. C comes out of every cell that sees both AC and BC.
- Unique Rectangle
- Four cells on a rectangle across two boxes can't all hold only the same two digits, because the puzzle would then have two solutions.
There are harder techniques still, such as chains and forcing nets, used mostly for extreme puzzles.
How to learn them
- One at a time. Learn a technique, then play until you spot it without hunting for it.
- In order. Each technique is easier to see when the ones before it are automatic.
- With notes. From pairs on, every technique is found in the candidates, so keep them written down and current.
Questions
What is the best Sudoku technique to learn first?
Scanning for hidden singles. Take a digit, find a box that doesn't have it, and rule out the cells whose row or column already holds it. Easy puzzles need little else.
Do I need advanced techniques to solve hard Sudoku?
Sometimes. Many hard puzzles fall to pairs and locked candidates once your notes are complete. The advanced patterns come in when those run out.
What order should I try techniques in?
The order on this page: singles, then pairs and triples, then locked candidates, then the advanced patterns. After any step that changes something, start again from singles.
Learn them with the Smith
Ninesmith Sudoku is a calm Sudoku app for iPhone. The Apprenticeship teaches the rules and the singles in two short lessons, the Pattern Trainer drills the techniques on this page one at a time in sessions of 30 to 90 seconds, and when you're stuck the Smith names the technique and shows where to look.
Get the appFree, from the Apprenticeship to triples in the trainer. The four advanced techniques are in Premium, with the first scenario of each free. No ads in any tier.