How to play

Every puzzle has exactly one solution, and you can reach it by logic alone. You never need to guess.

The rules

Select a cell, then press a number key to place that digit. The arrow keys move the selection.
Turn on Notes to pencil in candidates instead. Press a number again to remove its note.
Erase clears your digit from the selected cell. The starting digits can’t be changed.
A digit that clashes with another in its row, column or box is underlined in red.
On the Play page, Standard counts your mistakes and shows a timer. Relaxed shows neither. The site remembers your last choice.
Pause hides the board until you resume.
Hint opens a short ladder of help. It starts with where to look, then names the technique, then explains why it holds, and the last step gives the answer. More help goes up a step and Skip to answer goes straight to the end. If a digit you placed is wrong, the ladder has three steps and starts by telling you so. Press H for a hint and Esc to close it.

Each difficulty adds techniques to the ones before it. Hints name the technique that finds each step.

Naked single: A cell where only one digit is still possible, because every other digit already appears in its row, column or box.
Hidden single: A digit that fits in only one cell of a row, column or box, even if that cell has other candidates.

Naked pair: Two cells in one row, column or box that share exactly the same two candidates. Those digits must go in those two cells, so they can be removed from the rest of the unit.
Hidden pair: Two digits that can only go in the same two cells of a unit. Every other candidate can be removed from those cells.
Pointing pair: When a digit’s candidates inside a box all lie in one row or column, the digit can be removed from the rest of that row or column outside the box.
Box/line reduction: When a digit’s candidates in a row or column all lie inside one box, the digit can be removed from the rest of that box.

Naked triple: Three cells in a unit whose candidates, taken together, are only three digits. Those digits can be removed from the other cells in the unit.
Hidden triple: Three digits that can only go in the same three cells of a unit. Every other candidate can be removed from those cells.
X-Wing: When a digit has exactly two possible cells in each of two rows, and those cells share the same two columns, the digit can be removed from the rest of both columns. The same works with rows and columns swapped.

Swordfish: The X-Wing across three rows: when a digit’s candidates in three rows all fall within the same three columns, it can be removed from the rest of those columns.
XY-Wing: A cell with candidates X and Y sees one cell with X and Z and another with Y and Z. Whichever digit the first cell takes, one of the other two must be Z, so Z can be removed from every cell that sees both of them.