The correlative conjunctions both . . . and . . . are best served by parallelism, which is easily achieved but just as easily overlooked. The conjunctions should be carefully positioned and their conjoined elements should be well balanced. That is, what follows both and what follows and should have the same grammatical form:
He was determined both to beat the record and to win over the crowd.
Here, the conjunctions frame two infinitives, which brings symmetry to the sentence – unlike the following constructions, which lurch rather awkwardly:
He was both determined to beat the record and to win over the crowd.
He was determined both to beat the record and win over the crowd.
Again, balance in the following sentences enhances their efficiency and euphony:
The game is suitable for both children and adults.
The game is suitable both for children and for adults.
The game is suitable both for children and adults.
The game is both suitable for children and adults.
In the last two examples, both and and do not carry equal weight, and the sentences become unbalanced. Although the sense is not destroyed, the rhythm is upset and logic is undermined. Following both with a preposition will lead readers to expect another preposition after and, while following both with an adjective will lead readers to expect another adjective after and, e.g.
The game is both suitable for children and enjoyable for adults.
But this is not what is being said, and the expectation – even if only momentary – may disorient the reader.
Recently I came across an example in Gerald Durrell’s Encounters With Animals:
“Once [the male trapdoor spider] has lifted the trapdoor and entered the silken shaft, it is for him both a tunnel of love and death.”
Most readers will automatically accept Durrell’s intention that the spider’s silken shaft (no sniggering please) is (1) a tunnel of love, and (2) a tunnel of death, but the sentence as it is written suggests that the shaft is (1) a tunnel of love, and (2) death. Better to have written:
It is for him a tunnel of both love and death.
Or, less plainly but more dramatically:
It is for him both a tunnel of love and a tunnel of death.