Learn Sudoku, one technique at a time
Nine lessons, in the order the techniques actually become necessary. Each one gives you a puzzle that requires its technique — not a random grid, but one picked because its solution cannot be reached without the deduction the lesson just taught you.
- Last free cellOne square left in a unit means the digit that unit is missing belongs in it.
- Cross-hatchingSweep one digit across the grid and let the rows and columns you already have rule out squares for you.
- Naked singleOne square, one surviving candidate. The most direct placement in Sudoku.
- Hidden singleThe only square in a row, column or box that can hold a digit — even when it still shows other notes.
- Naked pairsTwo squares, the same two candidates: those digits belong to them, so erase them everywhere else in the unit.
- Hidden pairsTwo digits that can only go in the same two squares — so everything else in those squares goes.
- Pointing pairsA digit cornered onto one line inside a box clears that line in the two neighbouring boxes.
- Box/line reductionThe same overlap argument run the other way — from the line into the box.
- X-WingFour squares in a rectangle that force a digit into two columns, clearing both of them elsewhere.
How the puzzles are chosen
Rarely does a random Sudoku need a specific technique. Measured over four thousand grids, fewer than one in a hundred requires box-line reduction, and about one in two hundred requires an X-Wing. Picking a grid at random and hoping it teaches the right lesson does not work.
So each lesson's puzzles were found by solving candidate grids the way a person would — technique by technique, in this order — and keeping only those where the lesson's technique genuinely fired. Every grid here has one solution and needs no guessing, which is true of every puzzle on this site. Each lesson also reports what its own grid asked for — how many deductions it takes, where its technique first becomes necessary, and what else it calls for along the way.
On the board, the hint button names the technique that applies to the square you are on rather than simply filling it in, so it can be used to check your reasoning instead of replacing it. That is the same solver these puzzles were selected with.
If you are starting from scratch
Read how to play first — the rules take five minutes. Then begin at lesson one. If you would rather read all the techniques as reference than work through them, the techniques index covers the same ground plus pencil notes and colouring.
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