We discuss the conversion of a pure screw dislocation into a helical (single helix) dislocation through absorption of vacancies. These helical dislocations having some edge and mostly screw character can pair up through attractive interaction to form a double helix. This attractive interaction between two helices is derived through the edge component of mixed dislocations and from the presence of vacancy jogs. Recent results on the growth of GeS [Citation1] and PbS [Citation3] nanowires with Eshelby twist showed large disagreements between experimental and theoretical Burgers vectors for these crystal structures

which can be resolved by introducing the double helix of screw dislocations.