How to play sudoku
The rules, then the exact ladder of techniques Stillgrid uses to grade difficulty — from your first naked single to X-Wing.
The goal
A sudoku is a 9×9 grid split into nine 3×3 boxes. Fill every empty cell with a digit from 1 to 9 so that each row, each column, and each 3×3 box contains all nine digits exactly once — no repeats anywhere. A proper puzzle has exactly one solution, so you never have to guess: every digit can be reasoned out.
Notes (pencil marks)
Once the obvious cells are filled, you track candidates — the digits still possible in a cell — as small “pencil marks.” Notes are the backbone of every technique past the basics: pairs, pointing, and X-Wing are all patterns you spot in the candidates, not the solved digits. On Stillgrid you can enter pencil marks by hand, or tap auto-pencil to fill every cell's candidates at once, and undo/redo is always one tap away.
Play your first game
The best way to learn is to solve. Work through the opening of a real puzzle below — each step shows you the one cell you can fill next and why.
Look for a cell whose row, column, and box already use eight different digits — only one digit can go there. Place it, and the next forced cell appears. Repeat: scan rows, columns, and boxes for cells with just one possibility.
Beginning — what "Easy" means
Easy puzzles on Stillgrid are fully solvable with the two single-cell techniques below. If a puzzle never needs anything harder, the grader labels it Easy.
Naked single
A cell whose row, column, and box together already contain eight different digits has only one candidate left. That digit is forced.
In the highlighted cell, the digits 1, 2, 3, 4, 5, 6, 8, and 9 already appear in its row, column, or box — so the cell must be 7.
Hidden single
Sometimes a cell has several candidates, but within a whole row, column, or box only one cell can hold a particular digit. That digit is "hidden" among other candidates, but it's still forced into that one cell.
Across this box, only one cell can possibly hold a 4 — the other cells are blocked by a 4 elsewhere in their row or column — so that cell is 4.
Common questions
How do I start a sudoku?
Scan each row, column, and 3×3 box for a cell that can only hold one digit — a naked single — and fill it in. As you place digits, more singles appear. When the obvious cells run out, start tracking candidates as pencil-mark notes and look for pairs and pointing patterns.
What is a naked single and a hidden single?
A naked single is a cell whose row, column, and box already use eight different digits, leaving only one possible digit for it. A hidden single is a digit that can only go in one cell of a given row, column, or box, even if that cell still shows other candidates.
What do notes or pencil marks do?
Notes let you record the candidate digits still possible in a cell. They're the basis of every technique past singles — pairs, pointing, and X-Wing are all patterns in the candidates. Stillgrid can auto-fill every cell's notes for you with auto-pencil.