A novel logline is a single sentence that encapsulates the premise of a book, shows the main character, the central conflict and the stakes. In short, it's the core of the story, dug out from the myriad elements that make up a novel, buffed and polished into one shiny, diamond-bright sentence.
Originating from screenwriting, this practice is hugely beneficial to authors, too. Popular advice suggests creating a logline for the book you're writing before you begin drafting, because it will help you distill exactly what the book is about. It can be a guide for when you're lost in the drafting, a personal North Star that leads you to where the story is meant to be going.
Crafting one might sound easy, but it's far from. I've spent days crafting a logline for my novel, and I'm certain I'll keep revising it before I start work on the book in earnest. In my experience, doing a logline for various WIPs has showed me clearly which stories are well-constructed and ready to be fleshed out, and which ones are muddled and need more thought.
Here’s what I wrote for Princes, the main novel in the series I'm currently working on:
To seal an alliance to fight off an invading empire, two princes from rival countries must not only marry but also overcome their mutual dislike.
This isn’t perfect (looking at it now, a few months later, I’m itching to take a fresh stab at it), but it succeeds in conveying genre and premise (romance/arranged marriage), who the main characters are, the problem they’re facing (trusting one another) and the stakes (war/invasion).
Then I stumbled on something very fun on twitter. A user asked people to “poorly explain [their] fave book” and after I read a bunch of tweets and even tweeted about my favourite books, it occurred to me that it’d be fun to write one for Princes. This is what came to mind:
Newly married royals fail at getting a divorce.
I didn't expect that this off-the-cuff idea would revitalise my view on the novel so much. It offered me a new idea to work in (that the princes will hope they can get the marriage annulled once the danger of invasion passes), and it's made the story a great deal more exciting for me. Although you’ll find no mention of stakes and premise in this type of summary (well, the goal is to explain the story “poorly”, a truly liberating sentiment), it captures the fun aspect: two princes who’ll try to end the war so they can get rid of each other, but will, unfortunately, catch feelings in the process!
