As you circulate around your neighborhood bookstore or trawl the fiction section of Amazon, your eyes may sweep across the words “A Novel” on many a cover. “The Mars Room: A Novel,” “Little Fires Everywhere: A Novel,” “Purity: A Novel.”

“Duh,” you may think, though it’s possible the phrase won’t cause more than a faint blip on your radar, too small to register as a thought. As a book marketing convention, “A Novel” is so common that it hardly seems worth remarking on. When I started emailing people in the book publishing world to see if they could tell me why this tradition exists, I received multiple responses along the lines of, “I’m not sure I have much to say.”

I’m not the only one to pursue the question. When a book lives in the fiction aisle or is neatly tagged as such online, does it really need to announce itself as a novel? The answer to that is easy: We don’t always encounter books in the well-organized confines of a library, bookstore, or e-commerce algorithm. “A Novel” is not a subtitle but the reading line on a book cover, which explains its contents to a potential reader and serves as a useful signpost when you’re rooting through an unsorted stack of books. In theory it helps marketers sell books, by making their contents immediately known. This thinking seems to be particularly prevalent in the American market. When a book crosses the Atlantic from the United Kingdom, “A Novel,” is often added to its cover, as with the American edition of Sally Rooney’s “Normal People”; the original version does not have this distinction.

Thankfully, “A Novel” doesn’t exist solely for reasons as practical and boring as this.

Books have used the “XYZ: A Novel” format since the 17th century, when realistic fiction started getting popular. The term “novel” was a way to distinguish these more down-to-earth stories from the fanciful “romances” that came before, says Steven Moore, author of “The Novel: An Alternative History.” Then, as now, it was a tag that identified the kind of literature you were getting yourself into.

And Michael Schmidt, who wrote “The Novel: A Biography” offers this: Because so many novels of the 17th and 18th century adhered to historical truth in their settings, marking them as novels made it clear that they were in fact works of fiction and not real-life accounts.

In the early 20th century, with the rise of modernism in literature, the reading line started serving a new purpose. As the genre became more and more experimental, publishers put “A Novel” on book covers to reassure people that they were approaching something familiar, while simultaneously stretching the definition of what a novel could be. “Pale Fire” may have been a mashup of poetry, commentary, and prose, but its first edition, in 1962, still had the tagline “A new novel by Vladimir Nabokov.”

“It was a way of telling readers that the novel was more than just a realistic story — it could be something experimental,” says Moore. “It indicated that the novel was a growing genre.”

This use case has persisted. Moore recalls putting the words “A Novel” on book covers when he worked at a small press in the 1990s that published “a lot of experimental, odd-looking things.”

“We were careful to use the ‘A Novel” tag to show that the novel is an expansive thing. There’s always some smart aleck who says, ‘This isn’t a novel,’ as though the novel is narrowly defined,” he says.

As much as “A Novel” can be used to widen the genre’s horizons, today it’s also used to create a hierarchy, to set a work of fiction on higher ground than its peers. Norah Piehl, the executive director of the Boston Book Festival, has noticed that it’s often applied to novels “with certain pretensions to literary-ness.” These are not the romance novels or thrillers that you pick up at the airport, the covers of which are stylized in such a way that you’d never mistake them for anything else. They are books with abstract titles and beautifully spare cover art that aim to win awards, that get reviewed by the New York Times Book Review, that bequeath to their readers the cultural capital of being able to say “Have you read…?”

Piehl tallied up the numbers and told me that at the 2018 Boston Book Festival, 35 of the 50 featured works that would be classified as a novel had this reading line. Excluding romance novels and thrillers, none of which said “A Novel” on the cover, the proportion rose to more than 80 percent.

“Usually, gracing a new work of fiction with the subtitle ‘A Novel’ is a vaguely pretentious flourish, a way for authors to nominate themselves to the company of the 19th-century masters who regularly published new work with this semi-redundant act of genre specification,” begins Chris Lehmann’s Washington Post review of Stephen Glass’s “The Fabulist: A Novel.”

When it’s not making a statement about what literature is and is not, “A Novel” can serve a real, functional purpose. It’s useful when a writer wants to deny that they’re writing from life when that’s exactly what they’re doing, or when someone well-known for their nonfiction or criticism crosses into fiction.

“Last year we presented James Wood, the literary critic, for his novel, and I think in that case it is useful for the reader to be like, ‘Oh, this isn’t a work of literary criticism,’” says Piehl.

While frequently self-serious, “A Novel” can be used to winking effect, like George Singleton’s “Novel: A Novel,” A.J. Perry’s “Twelve Stories of Russia: A Novel, I Guess,” and Padgett Powell’s “The Interrogative Mood: A Novel?” which is written entirely in questions. Moore says that when he came across “The Care and Feeding of Ravenously Hungry Girls: A Novel” on Amazon, he interpreted the phrase as somewhat mischievous, as though anyone would confuse the book for a how-to guide.

“A lot of people have had fun with the term,” says Moore.

Cover designers often take a playful approach to reading lines, too. For “Exes,” designers Claire Williams Martinez and Charlotte Strick hand-lettered “A Novel” onto the cover art such that it appears to be trailing behind a car flying skyward. The words are tiny, a witty flourish rather than the meat of the composition. You could easily mistake them for a plume of exhaust.

Some contemporary designers pervert the hierarchy of information on a book’s cover by making “A Novel” just as big as the title and author name, which are more important than the reading line. “Immigrant, Montana,” designed by Janet Hansen, has all three in the same color and typeface; the alternate title “Lover, Bihar” has been crossed out, as has “A Meditation” below “A Novel.” Other designers use the fact that “A Novel” is a tertiary piece of information to their advantage, seeing it as an opportunity to use type in a more inventive way. For “A Loving, Faithful Animal,” Strick and Williams Martinez used the same typeface throughout but sliced “A Novel” in half, placing it at the top and bottom of the jacket to create a visual loop meant to represent the “inherited complexes” in families. You could never do that to an author’s name or a book’s title.

“Here, ‘A Novel’ was our friend,” says Strick. “It was helpful to have the extra type.”

For designers and marketers alike, “A Novel” is a tool that can be used in any number of ways and for any number of conflicting reasons, as wide-ranging and diverse as the genre it describes. People may not have much to say about it, but it does say a lot.