A double-crossover reverses the phase of the middle locus with respect to the flanking loci, but leaves the phase of the outside loci unchanged. Here, the parental strands (on the top and bottom) are v p⁺ b⁺ and v⁺ p b respectively. So,  p is trans to the v locus on the left, and cis to the b locus on the right. The phase relationships of the middle p locus are reversed by the double cross-over between the two inner chromatids, while the phase relationship of the outside loci remains trans.

This also means that the occurrence of a double-crossover between the outside markers is undetectable by inspection of their phenotypic expression.