Good and evil, e.g.
Else clues• 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
• (geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry
• A classification into two opposed parts or subclasses
• Being twofold