Parallelism makes a series both graceful and easy to follow.

In the eighteenth century, armed forces could fight in open fields and on the high seas. Today, they can clash on the ground anywhere, on the sea, under the sea, and in the air.

– Donald Snow and Eugene Brown, The Contours of Power

The parallel phrases, as well as the parallel structure of the sentences themselves, highlight the contrast between warfare in the eighteenth century and warfare today.

In the following sentences, note how the revisions make all items in the series parallel:

Items in a list, on a formal outline (7e), and in headings in a paper (9c) should be parallel.