Full House

If eight of the nine cells in a row, column or box are filled, the last cell must hold the digit that is missing. This is the simplest step in sudoku.

Example: row 4 contains 1, 2, 3, 4, 5, 6, 7 and 8, with one empty cell. That cell is a 9.

Notation

Full House: 1 goes in r4c5.