METHODS
Sudoku techniques: from single candidates to chains
Sudoku techniques are not isolated tricks. They all express the same constraint: one occurrence of each digit in every row, column and box.
Updated · AE Book Studio
Naked and hidden singles
A naked single is a cell with one remaining candidate. A hidden single is a digit with only one possible cell inside a row, column or box.
Repeat both searches after every significant elimination.
Locked candidates
If every possible occurrence of a digit in a box lies on one line, remove that digit elsewhere on the line. Conversely, if a row confines a digit to one box, other cells in that box exclude it.
This interaction between units unlocks many hard puzzles.
Pairs and triples
Two cells containing the same two candidates reserve those digits, allowing their removal elsewhere in the unit. A hidden pair begins with two digits confined to the same two cells.
The principle extends to triples, but every condition must be checked carefully.
X-Wings and chains
An X-Wing occurs when a candidate occupies the same two columns in two rows. Other instances in those columns can be removed. Chains then alternate strong and weak links between candidates.
Drawing the reasoning or using two colors reduces errors. After any elimination, return to simple scans.
FAQ
Frequently asked questions
Which technique follows singles?
Locked candidates and pairs are the most useful bridge to hard puzzles.
Must I know technique names?
No. Understanding and validating the constraint is enough.
What is an X-Wing?
A pattern across two rows and two columns that eliminates one candidate elsewhere.