Basics

The essential Sudoku rules and foundational solving techniques every player should know before moving on to advanced strategies.

Last Free Cell

When only one empty cell is left in a row, column or box, it must contain the only missing digit.

Sudoku cells ruled out until only one placement remains for the number 1

Last Remaining Cell

When only one cell in a row, column or box can accept a particular digit, that digit must go there.

A highlighted Sudoku row, column and box converging on the number 6

Last Possible Number

Look at the digits already present in a cell's row, column and box to find the only digit that can still fit.

A pencil entering candidate 4 among the small notes in a selected Sudoku cell

Notes in Sudoku

Use pencil marks to list candidate digits in a cell. Good notes make hidden singles, pairs and triples visible.

How the basic Sudoku techniques work

A standard Sudoku has nine rows, nine columns and nine 3×3 boxes. Each of these units—often called a house—must contain the digits 1 through 9 exactly once. Solving therefore starts with constraint checking: compare an empty cell with its row, column and box, then rule out every digit already present. No guessing or arithmetic is required.

The guides in this section teach the first three ways to place a digit. A Last Free Cell completes a house with one empty cell. A Last Remaining Cell finds the only position available for a particular digit in a house. A Last Possible Number finds the only legal digit for one cell after all three houses are considered. The notes guide then explains how to record candidates accurately—the essential working language for every more advanced Sudoku strategy in this reference.

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