Sudoku Rules: Rows, Columns and Boxes
Sudoku has one rule, applied three ways. In five steps on a real grid you learn it, enter your first digits and see what happens when two digits clash.
Step 1 of 5
Rows, columns and boxes
A sudoku grid has 81 cells: 9 rows, 9 columns, and nine 3×3 boxes marked by the thick lines. Some digits are given. You fill in the rest so that every row, every column and every box holds each digit from 1 to 9 exactly once.
The arrows run along row 3, down column 7 and across the middle box. Rows are counted from the top, columns from the left, and the boxes from 1 to 9 the way you read, so the middle box is box 5. The numbers along the top and left count them. These lessons name a cell by its row and column: r3c7 is row 3, column 7, where the row and column arrows cross.
Place your first digit
Select a cell by clicking or tapping it, then tap a digit on the pad under the grid. On a keyboard, type the digit; the arrow keys move between cells.
Row 3 has eight of its nine digits. The cell ringed in gold is its only gap.
Your move
Fill the ringed cell with the digit row 3 is missing.
Columns and boxes count too
The same rule holds for columns and boxes. The ringed cell r7c3 is the last empty cell of column 3. The ringed cell r4c5 is the last empty cell of box 5, the middle box. Their rows still have other gaps, so look along the column and around the box instead.
Your move
Fill both ringed cells.
Two digits that clash
Row 2 is missing a 3 and a 4, and both of its gaps are in box 3, so neither the row nor the box tells you which goes where. The columns do. Put a digit where its column already has one and the two clash: the grid marks them both, then takes yours back.
Your move
Fill both ringed cells without a clash.
A finished grid
This grid is solved. Every row, every column and every box holds 1 to 9 once each, and no two equal digits share a row, column or box.
How many 7s does it hold?
What you learned
- Each row, column and 3×3 box holds every digit from 1 to 9 exactly once.
- Select a cell, then tap a digit or type it.
- When a row, column or box has one gap left, the gap takes the digit it lacks.
- Two equal digits in the same row, column or box clash. Check all three before you place a digit.
- A finished grid has nine of each digit.
In this course
Unit 1The rules
- Sudoku Rules: Rows, Columns and Boxes
Unit 2Solving techniques
Take it to the table.
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