The Rules of Sudoku Explained for Complete Beginners

The board you are working with

A Sudoku board is one large square divided into 81 small cells, laid out as 9 rows across and 9 columns down. Those same 81 cells are also grouped into nine smaller squares of 3 cells by 3 cells, usually called boxes or regions. Rows, columns and boxes are the only three shapes you ever need to think about, and every rule in the game is built on them. It helps to picture the board with coordinates, so r1c1 is the top left cell, r1c9 is the top right, and r9c9 is the bottom right. That rNcN shorthand makes it easy to talk about exactly which cell you mean.

When a puzzle begins, some cells already hold a value. These are your starting clues, and they never change during the game. In Shapedoku we call them locked shapes or ancient marks, and they are drawn to look fixed so you are never tempted to edit them. Every other cell is empty, and filling those empty cells in correctly is the entire challenge.

The one rule that runs everything

There is really only a single rule, and it is simply stated three ways. Each row must contain every value exactly once. Each column must contain every value exactly once. Each 3 by 3 box must contain every value exactly once. In classic Sudoku the nine values are the numbers 1 to 9. In Shapedoku they are nine distinct shapes, but the logic is identical, because the game never cares what the symbols mean, only that there are nine of them and none may repeat.

Said even more plainly, no value is allowed to appear twice inside the same row, the same column or the same box. If you ever spot the same value twice in any one of those three groups, you know for certain that something earlier went wrong and needs fixing.

A worked example in one row

Suppose row 5 already holds eight different values and the only empty cell is r5c7. You list what row 5 already contains and find that every value is present except one. Because a row must hold all nine values exactly once, that single missing value is forced into r5c7. There is no choice and no guessing involved. That tiny move is Sudoku in miniature: you read what a group already has, work out what it still needs, and place the value that is the only legal option.

The same reasoning works across a box. If the top left box holds seven values and two cells are empty at r2c1 and r3c3, you check which values the box still needs, then check the rows and columns those two cells sit in. Often one of the two missing values is blocked from one cell by its row or column, which forces the other value into place.

What a finished, winning board looks like

You win when every one of the 81 cells is filled and all three rules hold across the whole board at the same moment. A completed Sudoku has each value appearing exactly nine times in total, once in every row, once in every column and once in every box. A well made puzzle has exactly one correct solution, which is why you never have to guess: there is always a path to the single answer through logic alone.

Common misunderstandings to drop early

• Diagonals do not matter in classic Sudoku. Only rows, columns and boxes carry the rule, so a repeat along a diagonal is completely fine.
• You never need arithmetic. Sudoku is about placement and deduction, not adding numbers up, which is why shapes work just as well as digits.
• There is no guessing in a properly made puzzle. Every single step can be reasoned out from what is already on the board.
• The starting clues are permanent. You only ever fill the empty cells, and you never change a given.

That short list clears up most of the confusion beginners carry into their first few games. Once you stop looking for diagonals and stop expecting math, the board becomes much calmer to read.

Why shapes make the rules easier to see

Shapedoku keeps the classic rules exactly and swaps the numbers 1 to 9 for nine glowing shapes such as a triangle, a hexagon, a circle and a heart. Each token is distinct by form as well as color, so the board stays readable even for players who see color differently. When you are learning the rules, shapes can actually help, because a repeated shape in a row jumps out visually faster than a repeated digit does.

The fastest way to make it click

You now know the whole rule set. If you understand rows, columns and boxes, and you accept that no value may repeat inside any of them, you already know how to play Sudoku. Reading about it only takes you so far, though. The rules truly settle once your own hand places a few pieces. Open an Easy board in Shapedoku, tap a cell, and drop in a shape, and within a minute the logic will feel completely natural. There is a free Beginner tutorial as well if you want the first few moves guided.