What is the metaphor contained in the title of The Left Hand of Darkness?

The title comes from one of the mythical tales enriching
the book; the complete sentence is: “Light is the left hand of darkness, darkness is the
right hand of light.” Light and darkness are obviously two contrasting elements which
are seen here as interrelated; the juxtaposition of seemingly different images or ideas
is clearly one of the themes of the novel and, especially, of the myths. We are reminded
of Blake’s Marriage of Heaven and Hell, where opposites are put
together to surprise and puzzle the reader; for example, one concept (good) cannot exist
without its opposite (evil), as “the Lamb” needs “the Tyger”. In LeGuin’s book, one
contrasting element defines the other, and one cannot survive without the other. Every
creature is endowed with contradictory components which, instead of crippling the
person, make him/her whole; the duplicity of all creation is underlined every step of
the way. This clearly refers to the inherent twofold nature of the Gethenians but also
to the relationship between Genly Ai and Estraven. The two find a profound understanding
in the story before and after Estraven’s death, which is a powerful - if sad - reminder
that we can come together as one even if we belong to distant worlds and that wars waged
against what is different are not necessary.