Sudoku glossary: terms and techniques explained simply

When you start playing Sudoku, the rules seem fairly simple. Then, as soon as you go deeper, many new terms appear: candidates, blocks, Naked Single, Hidden Pair, Pointing, X-Wing and many others.

This glossary is here to make everything clearer. You will find a simple explanation of the main terms used in Sudoku and in the Sudoku Arena guides, so you can check it whenever you come across a word or technique you do not remember.

The goal is not to memorize everything. It is to have an orderly reference, useful both for beginners and for anyone who wants to quickly review a technique.

A cell is the single space in the grid where a number or symbol can be entered.

In classic 9×9 Sudoku there are 81 cells. Some are already filled at the start of the game, while the others must be completed by the player.

Each cell always belongs to three areas at the same time: a row, a column and a block. To understand which number can be entered in a cell, you must check all three of these areas.

A row is a horizontal line of the grid.

In 9×9 Sudoku there are 9 rows, each made of 9 cells. When the grid is completed, every row must contain all numbers from 1 to 9 without repetitions.

If a number is already present in a row, it cannot be entered in another cell of the same row. This constraint is one of the fundamental checks to make before confirming a move.

A column is a vertical line of the grid.

In 9×9 Sudoku there are 9 columns, each made of 9 cells. Every column must also contain the numbers from 1 to 9 without repetitions.

When you analyze a cell, you must always check whether the number you want to enter is already present in its column. If it is already there, that number is not valid for that cell.

A block is one of the internal areas of the grid.

In classic 9×9 Sudoku, each block is made of 3 rows and 3 columns, so it contains 9 cells. The grid is divided into 9 blocks of 3×3.

When the grid is completed, every block must contain all numbers from 1 to 9 without repetitions. Many solving techniques, such as Hidden Single, Pointing and Claiming, use precisely the relationship between blocks, rows and columns.

In Sudoku, a unit is any area of the grid that must contain all the required symbols without repetitions.

In classic 9×9 Sudoku, the units are rows, columns and 3×3 blocks. When a technique refers to the “same unit”, it means that the involved cells belong to the same row, the same column or the same block.

A line is a row or a column of the grid.

The term is used especially in techniques that connect blocks, rows and columns, such as Pointing and Claiming. When we talk about eliminations “along a line”, we mean along a row or along a column.

A bivalue cell is an empty cell that has exactly two possible candidates.

Bivalue cells are important in some advanced techniques, such as Y-Wing, because they allow reasoning based on precise alternatives.

A candidate is a number, or more generally a symbol, that could be entered in a cell without breaking the rules of Sudoku.

For example, if a cell cannot contain 1, 2, 3, 5, 7 and 9, the remaining candidates might be 4, 6 and 8.

Candidates are not certain numbers. They are still-valid possibilities. During the solve, some will be eliminated and others will become certain solutions.

Candidates are essential in medium and hard Sudoku puzzles, because many techniques are based precisely on their distribution.

A note is the visual representation of candidates inside a cell.

In paper Sudoku, notes are often written in small size. In online Sudoku, they can be managed manually or automatically by the site.

Notes help you avoid having to remember all possibilities mentally. To be useful, however, they must be updated every time you enter a number in the grid.

A technique is a logical method that allows you to enter a number or eliminate candidates.

Techniques are used to solve Sudoku without entering numbers that lack a logical justification. Each technique has a precise rule and produces a verifiable consequence.

Some techniques are very simple, such as Naked Single and Hidden Single. Others are more advanced, such as X-Wing and Y-Wing.

On Sudoku Arena, the techniques covered in the Learn section can also be used to explain hints and help the player improve.

Naked Single is a basic technique.

It occurs when a cell has only one possible candidate. The other numbers have been excluded by the grid constraints or by previous logical eliminations.

In this case, you can enter the remaining candidate with certainty.

Simple example: a cell can contain only 7. Then that cell must be 7.

Hidden Single is a basic technique.

It occurs when a number can be entered in only one position within a row, column or block.

The cell may also have other candidates, but that number is forced because in the considered area it has no other possible positions.

Example: in a block, the number 5 can go in only one cell. That cell must be 5.

Naked Pair is an intermediate technique.

It occurs when two cells in the same row, column or block contain exactly the same two candidates.

Those two numbers must occupy those two cells, so they can be eliminated from the other cells in the same unit.

Example: two cells in the same row contain only {2,8}. You can eliminate candidates 2 and 8 from the other cells in the same row.

Hidden Pair is an intermediate technique.

It occurs when two numbers can appear only in the same two cells of a row, column or block.

The involved cells may also contain other candidates. In that case, you can remove from those two cells all candidates that are not part of the pair.

Example: in a column, numbers 3 and 9 can appear only in two cells. Those cells are reserved for 3 and 9.

Pointing is an intermediate technique based on the relationship between blocks and lines.

It occurs when, inside a block, all possible positions of a certain number are on the same row or the same column.

In this case, that candidate can be eliminated from the other cells of the same row or column outside the block.

To remember it: Pointing starts from the block and points toward the line.

Claiming is an intermediate technique similar to Pointing, but it starts from the opposite direction.

It occurs when, in a row or column, a candidate can appear only in the cells that belong to a specific block.

In this case, that candidate can be eliminated from the other cells of the block.

To remember it: Claiming starts from the line and restricts the block.

Naked Triple is an intermediate technique.

It occurs when three cells in the same unit - row, column or block - collectively contain only three candidates.

Those three numbers are reserved for those three cells and can be eliminated from the other cells in the same unit.

The three cells do not necessarily have to contain all the same candidates. What matters is the overall set of possible numbers.

X-Wing is an advanced technique.

It occurs when a candidate is limited to the same two columns in two rows, or to the same two rows in two columns. The four positions form the corners of a logical rectangle.

If the candidate appears only in the same two columns in two rows, you can eliminate it from the other cells in those columns. If it appears only in the same two rows in two columns, you can eliminate it from the other cells in those rows.

X-Wing is useful in hard Sudoku puzzles, when simpler techniques are no longer enough.

Y-Wing is an advanced technique based on three cells, each with two candidates.

The main cell is called the pivot. The other two cells are the wings. The pivot sees both wings, while the wings share a common candidate that is not present in the pivot.

That common candidate can be eliminated from cells that see both wings.

Y-Wing requires attention because it is not based on a rectangle, but on a small logical chain between bivalue cells.

A well-built Sudoku grid should have only one solution.

This means that, starting from the initial numbers, there is only one correct way to complete the entire grid.

A unique solution is important because it makes Sudoku a true logic puzzle. If a grid had multiple solutions, some cells might not be deducible with certainty.

An error is a number entered in a cell where it cannot go, or an unjustified choice that pushes the grid toward a wrong solution.

It can be a number that creates an immediate repetition in a row, column or block, or a number that seems possible but leads the grid toward a wrong solution.

To avoid errors, every move should be based on precise reasoning. If you cannot explain why a number is correct, it is probably not time to enter it yet.

A hint is help that points you to a possible move or an available technique.

A hint is most useful when it does not simply give the answer. It should explain the reasoning: which technique is being used, which cells are involved and why the move is valid.

On Sudoku Arena, hints can become a learning tool, especially if you use them to understand the technique and not only to complete the grid.

The difficulty of a Sudoku indicates how complex the logical path needed to solve it is.

It does not depend only on the number of empty cells. Two grids with the same number of clues can have very different difficulty levels.

Difficulty depends mainly on the techniques required. An easy Sudoku can be solved with Singles. A medium Sudoku may require pairs, Pointing, Claiming or Naked Triple. A hard Sudoku may require X-Wing or Y-Wing.

Knowing the meaning of the main Sudoku terms helps you read the grid better, understand techniques and make better use of hints.

You do not need to learn everything right away. You can use this glossary as a reference whenever you encounter a new term or want to review a technique.

The more familiar you become with these concepts, the easier it will be to follow explanations and improve in later games.