Oak squares materialize the relative position of pieces in order to help visualize in what cases the knight threatens the opponent.

3 squares

2 squares

3 squares

4 squares

5 squares

From these topologies can be deduced all the cases where the knight can perform a fork.

2-square fork

3-square fork

4-square fork

5-square fork

2 squares

3 squares

4 squares

5 squares