Sudoku guide
How to play Sudoku
The rules of Sudoku in plain words, as the app plays them.
The goal
Fill the grid so that every row, every column and every 3×3 box holds the digits 1 to 9, each exactly once.
Givens
The digits printed at the start are the givens. They never change; you fill in the rest.
Rows, columns and boxes
Every cell sits in one row, one column and one box, and shares them with 20 other cells. A digit can't repeat among them, so each digit you place rules it out of those 20 cells.
Candidates and notes
A cell's candidates are the digits it could still hold: the ones not already in its row, column or box. Players write them small in the cell as notes, and cross them out as the grid fills in.
One answer, found by logic
Every puzzle has exactly one solution and can be solved by logic alone.
You never need to guess. Each technique is a way to prove that a digit must go in a cell, or that it can't.
Levels
A puzzle's level is the hardest technique it takes to solve.
Easy needs only singles: the last cell left in a row, column or box, a cell with one candidate, or a digit with one place to go.
Medium adds locked candidates: a digit held to the cells where one box and one line cross.
Hard adds pairs, triples and the X-Wing.
Expert adds quads, the Swordfish, the XY-Wing and the XYZ-Wing.
Each level has a new puzzle of the day, the same for everyone that day.
Mistakes and hints
There are no lives and no mistake limit. A digit that repeats in a row, column or box is marked at once, and any move can be taken back.
Each puzzle comes with one free hint. It shows where to look, names the technique and explains why it works.