The three fundamental constraint units in Sudoku. Each unit must contain the digits 1-9 without repetition.

Row, Column, and Block (also called Box) are the three constraint units in Sudoku. A 9x9 grid contains 9 rows, 9 columns, and 9 blocks (3x3), totaling 27 units. The rule of Sudoku is to place digits 1-9 without repetition in each unit.

Interaction Between Units

Each cell belongs to one row, one column, and one block, simultaneously subject to three unit constraints. This triple constraint is the source of candidate elimination power. Cells at intersections where constraints overlap tend to have fewer candidates.

The Concept of Peers

Cells sharing the same row, column, or block with a given cell are called its "peers." Each cell has 20 peers (8 from the row + 8 from the column + 4 from the block). Any digit placed in a peer is eliminated from that cell's candidates.