The everyday spoken language of a particular group, region, or time period - how real people actually talk versus how textbooks say they should.
Vernacular is the natural, informal language that a community actually uses day to day, as opposed to formal, standardized, or literary language. It includes slang, idioms, regional expressions, and speech patterns specific to a place, era, or social group. In fiction, using vernacular means writing dialogue (and sometimes narration) that sounds like real people from a specific world rather than characters reciting from a grammar handbook.
Your characters live in a world, and that world has a language. When a character from 1920s Harlem speaks the same way as a character from modern-day suburban London, something's wrong. Vernacular is how you root characters in time, place, and community. It's the difference between dialogue that feels like it was written and dialogue that feels like it was overheard.
Celie's letters are written in African American Vernacular English, and the vernacular is the voice - inseparable from the character and the story's emotional power.
Burgess invented an entire vernacular (Nadsat) for his teenage characters, blending Russian and English to create a speech pattern that distances the reader from the violence while immersing them in the world.
Mattie Ross narrates in a formal, slightly archaic vernacular that perfectly captures her time period, her upbringing, and her no-nonsense personality all at once.
Once you establish a character's speech patterns, maintain them throughout. Keep notes on each character's vocabulary and rhythms.
Vernacular is a fully functional language system with its own grammar rules. Characters who speak in vernacular can be brilliant, articulate, and complex.
Historical fiction is a balancing act. Use enough period language to create flavor, but remember your actual reader is in the present. Context clues help more than footnotes.
Write a one-page scene set in a place you know well - your hometown, your campus, your workplace. Have two characters talk the way people there actually talk, including slang, local expressions, and speech rhythms. Then hand it to someone unfamiliar with that place and ask if they can follow the conversation and get a sense of where it's set.