- A Sudoku grid has 27 units: 9 rows, 9 columns, 9 boxes (also called blocks or nonets)
- Every unit must contain each digit 1–9 exactly once
- Every cell belongs to exactly 3 units at the same time
- "Block," "box," "region," and "nonet" all mean the same thing: the 3×3 square
- Knowing which units share a cell is the foundation of every elimination technique
Rows: The Horizontal Groups
A row is a horizontal line of 9 cells. The grid has 9 rows, numbered 1 (top) to 9 (bottom). Each row must contain the digits 1 through 9 with no repeats. When you place a digit in any cell, that digit is eliminated from all other cells in that row.
Columns: The Vertical Groups
A column is a vertical line of 9 cells. The grid has 9 columns, numbered 1 (left) to 9 (right). Like rows, each column must contain digits 1–9 with no repeats. A digit placed anywhere in a column cannot appear again in that same column.
Boxes, Blocks, Regions, and Nonets
The 9×9 grid is divided into nine 3×3 squares. These go by several names depending on the source:
| Term | Used By |
|---|---|
| Box | Most English-language apps and websites |
| Block | Many British publications and books |
| Region | Academic and mathematical literature |
| Nonet | Puzzle theory and competition circles |
| Square | Casual usage (ambiguous — also means a single cell) |
All five terms mean exactly the same thing. On this site we use "box." Each box must contain digits 1–9 with no repeats, just like rows and columns.
How the Three Units Interact
The power of Sudoku comes from the overlap. Every cell sits at the intersection of one row, one column, and one box. That means placing a digit eliminates it from up to 20 other cells across three different units simultaneously.
For example, if you place a 5 in the top-left cell (row 1, column 1, top-left box), the digit 5 is now forbidden in:
- All 8 remaining cells in row 1
- All 8 remaining cells in column 1
- All 8 remaining cells in the top-left box
Some of those cells overlap, so the actual number of cells affected is 20 (not 24). This simultaneous elimination is why even one confirmed digit can unlock several others in a chain reaction.
Using Units to Find Your Next Move
Every technique in Sudoku is built on analyzing one or more units. Hidden Singles work by finding a digit that can only go in one cell within a unit. Naked Singles work by finding a cell where only one digit is still allowed by all three of its units. Learn more in the full techniques library or start a puzzle and practice live.