Rows, columns and columns intersect with each other and create therefore restrictions to set the numbers.

Therefore not all numbers can be set into every cell. To solve more complicated sudokus, an overview is required to see what numbers are possible.

This is done by writing pencil marks into the individual cells. A sudoku can therefor be solved by writing all pencil marks into each cell and then erasing the ones that are no more possible until the sudoku is solved. An empty sudoku with pencil marks looks therefore as follows: