All items in a series should be in parallel form—all nouns, all verbs, all prepositional phrases, and so on. Parallelism makes a series both graceful and easy to follow.

In the eighteenth century, armed forces could fight in openfields 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 structure of the phrases, and of the sentences themselves, highlights the contrast between the eighteenth century and today.

Items in a list, in a formal outline, and in headings should all be parallel.