Sudoku techniques
Ten of these map to a named step our own solver actually takes; the worked example on each page is mined from a real generated puzzle, not drawn by hand. The last two — uniqueness and pencil marks — are foundations the rest build on.
Naked single
How to spot a naked single: a cell with only one candidate digit left. Worked example plus a board to try it.
Hidden single
How to find a hidden single: a digit that can only go in one cell of a row, column, or box.
Locked candidates: pointing
Pointing pairs and triples: when a digit confined to one box can be eliminated from the rest of its row or column.
Locked candidates: claiming
Claiming pairs and triples: the row/column-first mirror of pointing, eliminating candidates inside a box.
Naked pair
Two cells, two shared candidates: how a naked pair lets you eliminate those digits everywhere else they'd conflict.
Hidden pair
Two digits trapped in the same two cells: how a hidden pair reveals eliminations other digits can't see.
Naked triple
Three cells sharing three candidates between them, and how to use that to clear a row, column, or box.
X-Wing
The X-Wing pattern: a digit restricted to the same two columns in two rows, and the eliminations it unlocks.
Swordfish
Swordfish extends X-Wing to three rows and three columns — here's how to actually spot one on a real board.
XY-Wing
XY-Wing: three bi-value cells chained together to eliminate a candidate none of them touches directly.
Uniqueness basics
Why a proper sudoku has exactly one solution, and how that guarantee itself becomes a solving technique.
How to use pencil marks
How to take sudoku notes properly — when to mark candidates, when it's clutter, and how our notes mode works.