A proper edge k-colouring of a graph G = (V, E) is an assignment c : E → {1, 2, . . . , k} of colours to the edges of the graph such that no two adjacent edges are associated with the same colour. A neighbour sum distinguishing edge k-colouring, or nsd k-colouring for short, is a proper edge k-colouring such that e∋u c(e) = e∋v c(e) for every edge uv of G. We denote by χ ′ (G) the neighbour sum distinguishing index of G, which is the least integer k such that an nsd k-colouring of G exists.