What are the similarities and differences between a radius and a diameter?

Similarities:

They are both line segments. They are both found in circles.Radius is a half of diameter and diameter is twice the radius making them dependent.Their intersection is always contains the center of the circle.Their set intersection equals to radius and their set union equals to diameter.

Differences:

Diameter is longer (unless its 0)Diameter touches two different points on the circumference while radius only one pointCenter angle of diameter is 180° while of radius is 0° or 360°.The Diameter is constructed via lineThe Radius is constructed via line or ray