Grammar related to “not only … but also…”

grammar

I know when using not only … but also … we should follow parallelism. Ex: not only cats but also dogs, or not only run but also jump. However, I am confused which of the followings is correct and is it necessary to put comma before the but?

  1. Generally, a system is required to be not only stable but also stable by some margin of safety.
  2. Generally, a system is required not only be stable but also be stable by some margin of safety.

Best Answer

This is not a real parallelism, as the second part of this structure does not offer another element, but simply adds a circumstance to the already existing element. Not only can also be used before the verb. To emphasise that this is an addition and not a parallelism, I would say:

Generally, a system is not only required to be merely stable, but also be stable by some margin of safety. [you omitted this "to" in your second example]

This article shows the way this correlating conjunction is used and says that not all grammarians are so strict about it being used for parallelism only.

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