The paired conjunctions both, ..and, either…or, neither…nor, and not only…but also require parallel structures.

e.g.         
– the lecture was both informative and enjoyable
– either the history exam or the physic exam is on Tuesday
– the missing papers are neither on his desk nor in the file
– he visited not only his cousin but also his grandmother

Pattern    :  
Both                                                    and
Either              (same structure)       or                    (same structure)
Neither                                               nor
Not only                                             but also