The structure of all-trans-hexatriene. The central double bond must be Z-configured. There is no way an E-configured central double bond could give an electrocyclic reaction. Do far so good. That leaves the question as to the conformations about the C-C single bonds. The trans refers to the conformation about the C-C single bond. And now you are seeing again the point that is critical for the pericyclic reactions: In pericyclic reactions it is customary to write the hexatriene in a way that suggests that the two double bonds are on the same side of the central double bond. Yet, the best structure of hexatriene is the all-trans-structure shown.