They are the children who are supposed to look alike, dress alike, behave alike and carry their almost supernatural closeness to their graves; twins, two babies from the same egg who have a bond that outsiders cannot break and who, consequently, live for each other.
But, as Penelope Farmer's exhaustively-researched anthology on the subject shows, that judgment comes partly from the fascination, and in some parts of the world, horror, that twins hold for outsiders. When it does hold true, the consequences are not as cosy as we would imagine; many twins have a love-hate relationship in which hate has the upper hand.
First, you must define your twin. Identical twins are, Larousee observed in Science of Life, 'the same individual of whom two copies have been printed', or 'one river flowing in two beds' according to Anatoly Pristavkin in The Inseparable Twins.
So much for the snap conclusions. Science proves more interesting. 'A study of twins reared apart has reported several cases where genes seem to be controlling not just broad tendencies, such as being intelligent or outgoing, but surprisingly detailed aspects of personal behaviour,' Jerome Burne wrote in The Times of London in 1993.
'For some time now the study [by the University of Minnesota] has been finding twins, separated at birth and reunited only in middle age, shared such curiously specific things as: being brilliant storytellers, refusing to express controversial opinions, being habitual gigglers, and always wearing seven rings.
'There was even a pair who always entered the sea backwards and then only on their knees.' Then there are fraternal twins - separate eggs, same womb - two individuals born at the same time, but with different characteristics; 'I must trust the substance, not the shadow; you are most like me, but are not the same,' wrote William Rider in the 17th century.
After getting the basics out of the way, offering some startling statistics as in the case of the woman who gave birth to 44 children in 33 years, including 13 pairs of twins and six sets of triplets, and discussing famous childhoods - the criminal Kray twins in Britain, the Dionne quins in Canada - Farmer moves into deeper waters.