The diagonals of a parallelogram bisect each other.

Try this Drag the orange dots on each vertex
to reshape the parallelogram. Notice the behavior of the two diagonals.

In any
parallelogram,
the diagonals (lines linking opposite corners)
bisect each other.
That is, each diagonal cuts the other into two equal parts.
In the figure above drag any
vertex
to reshape the parallelogram and convince your self this is so.