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Sudoku Glossary

Every sudoku word you’re likely to meet, in plain language. The board terms are shown on a real grid, and the technique terms link to their animated lessons.

The board

Grid

The whole 9×9 puzzle: 81 cells in nine rows, nine columns and nine blocks. Also called the board.

Cell

One of the 81 spaces. Each one ends up holding exactly one color. Also called a square.

Row

Nine cells across. Every row holds each color once.

Column

Nine cells down. Every column holds each color once.

Block

One of the nine 3×3 squares with the thicker borders. Every block holds each color once. Also called a box, region or nonet.

Unit

Any row, column or block. The rule of sudoku is the same for all three: nine cells, nine colors, no repeats. Also called a house.

Peers

The 20 cells that share a row, column or block with a cell. A cell can never match any of its peers. Advanced patterns like the XY-Wing are built on cells that are peers.

The puzzle

Clue

A bubble that’s already on the board when the puzzle starts. Clues can’t be changed. Also called a given.

Candidate

A color a cell could still hold. Notes are small dots that track candidates. Also called a note or pencil mark. Notes lesson

Bivalue cell

A cell with exactly two candidates left. Many advanced patterns are built from them.

Unique solution

A proper sudoku puzzle has exactly one answer, so every move can be proved without guessing. The rules

Singles

Full House

A row, column or block with one empty cell. The missing color goes there. Also called last free cell. Lesson

Obvious Single

A cell where only one color fits. Also called naked single, sole candidate or last possible number. Lesson

Hidden Single

A color that has only one possible cell left in a row, column or block. Also called last remaining cell. Lesson

Patterns

Obvious Pair

Two cells in one unit that hold the same two candidates and nothing else. Those two colors can be removed from the rest of the unit. Also called naked pair. Lesson

Hidden Pair

Two colors that can only go in the same two cells of a unit. Every other candidate can be removed from those cells. Lesson

Obvious Triple

Three cells in one unit whose candidates are only three colors between them. Also called naked triple. Lesson

Pointing Pair

When a block can fit a color only along one row or column, the color must go there inside the block. Rule it out of the rest of that row or column. Also called locked candidates. Lesson

Claiming Pair

When a row or column can fit a color only inside one block, the color must go in that block. Rule it out of the rest of that block. Also called box/line reduction. Lesson

X-Wing

When a color fits only two cells in each of two rows, and those cells share the same two columns, it can be ruled out of the rest of both columns. Join the four cells corner to corner and they make the X it’s named for. Lesson

Swordfish

The X-Wing idea stretched to three rows and three columns. Lesson

When you’re stuck

Test a Bubble

When a cell is down to two colors, try one and follow the moves it forces. If the board breaks, the other color is proved. Then undo the test. Also called trial and error or bifurcation. Lesson