DUALITY

(geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry

• Good and evil, e.g.

• Twoness

• Yin and yang, e.g.

• (physics) the property of matter and electromagnetic radiation that is characterized by the fact that some properties can be explained best by wave theory and others by particle theory

• A classification into two opposed parts or subclasses

• Being twofold