1 · The grid
Sudoku is played on a 9×9 grid of 81 cells. Some cells arrive already filled; the goal is to fill every empty one with a digit from 1 to 9. That is the entire task — what makes it a puzzle is where each digit is allowed to go.
2 · Rows
The grid has nine rows, running left to right. Each row must end up containing every digit from 1 to 9 exactly once — no digit repeated, none missing. The washed row below already holds 9, 3, 8, 1, 7, 6, 4, and 2; because a row needs all nine digits, its one empty cell can only be the missing 5.
3 · Columns
The nine columns, running top to bottom, obey the same law: every digit from 1 to 9 exactly once in each. Rows and columns cross, so one placement always answers to both — the washed column here is complete except for a single cell, and reading down it, the digit it still lacks is 2.
4 · Boxes
The bold lines cut the grid into nine 3×3 boxes, and each box is a house of its own: every digit from 1 to 9 exactly once inside it. This third constraint is what gives Sudoku its depth — a digit has to satisfy its row, its column, and its box all at the same time.
5 · Givens and your entries
The digits printed in ink are givens — the puzzle's fixed clues. They never move, and every good puzzle places them so that exactly one finished grid is possible. The lighter, pencil-grey digits are entries: cells you have filled yourself. An entry is correct when it breaks no row, no column, and no box — and if you change your mind, only entries can be erased.
6 · Notes
The small digits tucked into a corner grid are notes, also called pencil marks or candidates: the digits a cell could still legally hold. Keeping them honest — adding what is possible, striking what is no longer — is the whole craft of solving. Every technique on the ladder ahead, from the first single to the jellyfish, is a way of reading notes and narrowing them down.
This is the same stable teaching board the app's interactive Rules walkthrough uses — six steps on a live grid, with the washes drawn as you tap through.
