There are quite a few different writing style guides. MLA, APA, Chicago, etc. 

Most undergraduate programs teach MLA for their writing classes. MLA is commonly used for academic publications. If you go to graduate school, particularly for industry-based writing, you will most likely learn APA formatting. AMA is for medical documents, and Chicago is designed for writings in history and the humanities. It is less so used for sciences, but it isn't too rare to see scientific papers written in the Chicago style. 

Turabian style is for academic publications such as research papers, theses, and dissertations.

Style guides inform writers on how to format their writing. They provide details on how your document should look, from the cover pages to the line spacing to the page numbers. They also provide details on punctuation and grammar. For example: you will use Oxford commas in APA style, while AP style, which is for newspapers, magazines, and other press outlets, does not require the usage of the Oxford comma. 

Fiction is where the phrase "there are no right rules in writing" really comes into play. Style guides exist to help you stay consistent with your writing, and it's important to be consistent in fiction with things like how you write numbers: spell out numbers that can be written in one to two words with a hyphen, as suggested in MLA. You may use numerals for large numbers like calendar years, even if it's at the beginning of a sentence. Etc.

But not every piece of fiction follows the same style guide. Take The Road by Cormac McCarthy, for example, which does not use quotations for its dialogue or even breaks up its dialogue into separate paragraphs for each speaker. This dialogue feels like it is coming from two incredibly weary travelers. The formatting makes their speech sound tired, desperate, and a little hopeless. This style is faithful to the characters and it helps set the tone for the desolate environment they are living in (I ended that sentence with a preposition. Traditional grammar forbids starting and ending sentences with prepositions. Fiction allows you to start and end sentences with prepositions when it is in your authorial voice or the character's voice to do so...see what I mean? There are no set rules). It does not follow a style guide, but it is consistent with the story itself. Sometimes stories require authors to create their style guides or to take variations on style guides that already exist. Writing fiction isn't always about following someone else's rules; it's more about doing what serves your story best. 

Sometimes, fiction will be written in a "house style:" a style specific to the publishing house and its editors. Sometimes an author will come up with their own style guide. Sometimes fiction will follow something similar to MLA or APA, but it will break the rules as needed to serve best the author's voice as well as their characters. There isn't just one style guide to follow, and there aren't any prescriptive sets of rules that everyone must follow either. This is very different to academic writing, where you have to follow style guides and grammar rules to pass or publish your work.

When querying, many presses will ask you to submit your manuscript using a formatting style called "Modern Manuscript Format" or "Classic Manuscript Format." This format won't tell you whether or not to use the Oxford comma, but it will show you how your manuscript should look once an editor receives it. I have put a link to this formatting style in my references.

This essay is based on Les Edgerton's Finding Your Voice where he discusses style in detail. I would highly recommend that book if you are confused about how to follow writing rules in fiction. I quite like his last essay where he discusses prescriptive advice in fiction: "One of the problems I've personally had with many of the how-to writing books I've collected over the years...was the absoluteness of the author in ladling out his or her wisdom. A tone, often, that proclaims authoritatively, "I am the Way, the Truth, and the Light"' (Edgerton, 2003, p. 224). Edgerton goes on to discuss why the craft of writing is more about following your instincts rather than someone else's advice.

In summary: there isn't a particular writing style guide you have to follow as a fiction writer. There isn't a specific set of grammar rules you have to follow either. There isn't a specific way you have to write your story. But I would be wary of people who say that there are rules that you must follow, and then they go out of their way to define what they THINK those rules are. 

I would also recommend turning off Grammarly when writing fiction because Grammarly is programmed for professional writing and it suggests things that override your authorial voice. 

Fiction isn't about writing something perfect by traditional rules or writing standards. It isn't even about providing escapism to readers or giving them what they want. Fiction is a medium we use to find the truth about the human experience, whether or not that truth is comfortable or disturbing. Humanity doesn't follow certain rules...it's messy, inconsistent, and oftentimes confusing. So why should the medium we use to understand it have to follow the rules, too? 

Tell me what you think in the comments.

References

Answers to Writing Questions. (n.d.). Should I write out numbers or use numerals? Gotham Writers. https://www.writingclasses.com/

Edgerton, L. (2003). Finding your voice. Blue Skies Press.

Klems, B. A. (2017, February 13). When do I spell out numbers? . Writer's Digest. https://www.writersdigest.com/write-better-fiction/when-do-i-spell-out-numbers

Shunn, W. (n.d.). Proper manuscript format. Shunn.net. https://www.shunn.net/format/classic/1/

Sitar, D. (2020, May 8). What is a writing style guide, and which one should you use? The Write Life. Retrieved November 5, 2023, from https://thewritelife.com/writing-style-guide/