The book offers a unique in-depth understanding of the twin relationship, and the way in which twin development is affected by our attitudes to twins and our enduring fascination with them. It explores our historical fascination with this subject and the origins of this excitement, how our perceptions of twins reflect our own longing for a perfect soul-mate, and the effect this personal projection has on the development in twins. It is a book written with the general reader in mind rather than "experts".Twins share a deep psychic bond that forms the core of their twinship, but they are never identical. Many factors will affect their development, including the early mutual resonances and sensate experiences between them, and parental and societal attitudes in raising them. I explore the internal dynamics of the twin relationship and the factors that crucially affect and shape it: the nature of the internal twin bond; the twins' relationships with mother and father and siblings, including their perceptions about the twin relationship; the twins perceptions of each other within the intricately bound twin relationship; each twin's perception of the other's relationship with mother and father; and the outside world with its expectations and strictures on twins. The importance of enabling twins to develop individually within the twin relationship rather than remain as a bound pair, is examined within the context of the factors that influence this development. Both our unconscious perceptions about twins, and the way these perceptions affect our behaviour and attitudes towards the twins, will influence the individual development in twins and our treatment of twins in society. The way twins are used as a device in literature offers us a further opportunity to understand our own pre-conceptions about twins and doubles, and it affords confirmation of our understanding about twin relationships. The section on cultural practices looks at how the response to the birth of twins differs around the world according to the local myths, religious and the unconscious beliefs upon which they are based.