Sudoku Rules &
Solving Strategies

From core board mechanics to 15 master-level logical deduction techniques. Illustrated step-by-step with clean board diagrams and detailed walkthroughs.

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The Foundations

What is Sudoku and what are the rules of this game?

Sudoku is a popular logic puzzle with numbers. Its rules are quite simple, so even beginners can handle the simple levels.

1

9x9 Grid Layout

The classic Sudoku board consists of 81 cells organized into 9 rows, 9 columns, and nine 3×3 blocks.

2

Use Numbers 1 through 9

Only single digits from 1 to 9 may be entered into any cell on the board. No zeros or negative numbers.

3

Unique 3x3 Blocks

Each of the nine 3×3 sub-grids must contain the numbers 1 to 9 exactly once without duplication.

4

Unique Columns

Each vertical column must contain all digits from 1 to 9 with no repeated numbers.

5

Unique Rows

Each horizontal row must contain all digits from 1 to 9 with no repeated numbers.

6

No Guessing Required

Every valid Sudoku puzzle has a single unique solution that can be deduced through pure logical reasoning.

7

Victory Condition

The game is successfully completed when all 81 cells are filled in strict adherence to every row, column, and block constraint.

The Golden Principle

Sudoku is entirely a game of pure deduction and logic. There is never any need to guess. Every single cell can be solved with confidence by eliminating candidates and applying the techniques below step-by-step.

15 Solving Techniques

Master Every Strategy

Explore step-by-step guides and high-resolution board illustrations.

Last free cell

Beginner4 diagrams

Last free cell

"Last free cell" is the basic Sudoku solving technique. It's pretty simple and based on the fact that each 3×3 block, vertical column or horizontal row on Sudoku grid should contain numbers from 1 to 9 and each number can be used only once within 3×3 block, vertical column or horizontal row.

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Last remaining cell

Beginner2 diagrams

Last remaining cell

"Last remaining cell" is another basic Sudoku strategy. It's based on the fact that numbers should not be repeated within 3×3 block, vertical column and horizontal row.

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Last possible number

Beginner2 diagrams

Last possible number

Last possible number is a simple strategy that is suitable for beginners. It is based on finding the missing number. To find the missing number you should take a look at the numbers that are already exist in the 3x3 block you are interested in, and in the rows and columns connected with it.

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Notes in Sudoku

Notes in Sudoku

If you get stuck on Sudoku grid and don't see the obvious solutions for the rest of cells, you should use Notes. With the help of Notes you should fill in all the possible options for each blank cell, focusing on the numbers that are already on the Sudoku grid.

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Obvious singles

Beginner2 diagrams

Obvious singles

This strategy is based on the correct placement of Notes. Sometimes it is called Naked Singles. The point is that in a specific cell only one digit (from the Notes) remains possible.

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Obvious pairs

Intermediate1 diagram

Obvious pairs

Like the "Obvious Singles" technique, "Obvious pairs" is based on the correct placement of Notes. The point is that you should find 2 cells with the same pairs of Notes within 3x3 block. This means that these pairs of Notes cannot be used in other cells within this 3x3 block. So they can be removed from your Notes. It will be easier to understand this strategy if you look at the example.

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Obvious triples

Intermediate3 diagrams

Obvious triples

This Sudoku solving technique is built upon the previous one - "Obvious pairs". But "Obvious triples" is not based on two numbers from the Notes, it's based on three. This is the only difference. To understand better, let's take a look at the example.

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Hidden singles

Intermediate2 diagrams

Hidden singles

"Hidden singles" is a quite simple Sudoku technique. The point of "Hidden singles" is that a Note is the only one of its kind in an entire row, column, or 3x3 block. However, this technique requires careful attention from the player, because it can be quite hard to spot the single Notes.

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Hidden pairs

Intermediate2 diagrams

Hidden pairs

"Hidden pairs" technique works the same way as "Hidden singles". The only thing that changes is the number of cells and Notes. If you can find two cells within a row, column, or 3x3 block where two Notes appear nowhere outside these cells, these two Notes must be placed in the two cells. All other Notes can be eliminated from these two cells.

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Hidden triples

Intermediate2 diagrams

Hidden triples

"Hidden triples" technique is very similar to "Hidden pairs" and works on the same concept.

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Pointing pairs

Intermediate2 diagrams

Pointing pairs

"Pointing pairs" applies when a Note is present twice in a block and this Note also belongs to the same row or column. This means that the Note must be the solution for one of the two cells in the block. So, you can eliminate this Note from any other cells in the row or column.

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Pointing triples

Intermediate2 diagrams

Pointing triples

"Pointing triples" technique is very similar to "Pointing pairs". It applies if a Note is present in only three cells of a 3x3 block and also belongs to the same row or column. This means that the Note must be a solution for one of these three cells in the block. So, obviously it can't be a solution of any other cell in the row or column and can be eliminated from them.

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Х-wing

Advanced2 diagrams

Х-wing

"Х-wing" is an advanced sudoku technique, which is based on the two parallel rows or two parallel columns. You shouldn't pay attention to the 3x3 blocks as they aren't involved in this strategy.

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Y-wing

Advanced2 diagrams

Y-wing

"Y-Wing" technique is similar to "X-Wing", but it based on three corners instead of four.

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Swordfish

Advanced4 diagrams

Swordfish

The "Swordfish" technique is an advanced Sudoku strategy. It's usually applied in the hard levels of Sudoku puzzles to eliminate candidates. "Swordfish" is similar to X-wing but uses three sets of cells instead of two.

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Put Your Logic into Practice

Ready to solve your next puzzle?

Apply these techniques on real Sudoku boards. Start from Easy levels and progress all the way to Master and Extreme.

Start Playing Now Lesson 1: Last Free Cell