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Understanding DNA supercoiling

While trying to understand DNA supercoiling, I came across these lines in the book Genetics by Ursula Goodenough :

All natural DNA is superhelical. The axis of the duplex itself follows a helical path in space. For virtually all DNA, the sense of the superhelical turns is opposite to that of duplex, thereofer, they are negative supertwists.

I am unable to understand these lines. Specially, the second one.