Andrew Rotilio knows what it feels like to not fit in. Born to first-generation Italian-Canadian parents, the Expanse actor’s first language wasn’t what his teachers spoke in school. He grew up speaking Italian, then learned French and English as a way of surviving, of blending in. By the time he mastered his fourth language, he wasn’t trying to navigate the social complexities of the real world—he was trying to leave it behind completely.

As the man who plays Diogo, Rotilio is as close as it comes to a native speaker of Belter, the constructed language of the oppressed working class on The Expanse. Developed by linguist Nick Farmer, with input from accent coach Eric Armstrong, the tongue is the patois of a group of people who survive by scavenging materials in the Asteroid Belt (hence, Belters), and Rotilio had to immerse himself in it to assimilate to Diogo’s way of life.

“The language is really at the center of who these people are. It changes you, it affects you,” says Rotilio. “It’s like when you put on a period costume and you play King Henry, and you become King Henry. It’s like that, but with the language.”

The Expanse takes place 200 years in the future, a time when humans have colonized Mars and the entire solar system. Belters are the displaced underclass, a great hoard of humanity who left every nation on Earth to find work in the outer reaches. And, just like their bodies changed to acclimate to low gravity, their language also evolved to communicate in the universe’s ultimate melting pot. Belter is the lingua franca for the universe’s most dispossessed peoples. To hear it spoken on the show is to understand how much has changed in this future, but also how similar the Belter experience is to that of immigrants at any time, including now.

This is perhaps why the language resonates so well with the show’s fans on Earth. Enthusiasts regularly tune in to Rotilio and Farmer’s weekly Belter class on Twitter. A punk band wrote a song in the language. And, according to Farmer, someone even proposed marriage in Belter. The Expanse’s patois has become, like Klingon and Dothraki, the show’s great unifier—the slang all devotees speak.

Creole, in Space

In the Expanse novels, Belter is mostly just a dialect. But when the show jumped to TV, the producers brought on Farmer to make a it a fully realized language. When he got started, Farmer immediately understood that Belter was a creole. Creoles are based on a mother tongue—in this case English—but incorporate the influence of many other languages. Farmer looked to Haitian Creole for inspiration. Nowhere near as many languages contributed to that creole as to Belter, but it was the best correlate on Earth because it developed after people from all over the world arrived on the island—in many cases by force. “The situation for the Belters is the same,” Farmer says, “but in space.”

Belter is composed mainly of Chinese, Japanese, Slavic, Germanic, and romance languages because Earth’s most common tongues would be the ones to survive to form the new brogue of the cosmos. And every choice Farmer makes about new inclusions affects the world-building of the show. If he puts in a Zulu word, that means there are Zulu people in the Belt. Knowing this, he took his time with the language, writing pages and pages of grammar and hundreds of vocabulary words, which he keeps track of in a Google Doc that he guards like his first-born child. Which, in a way, it is.

The Sound of Otherness

Farmer knew how he wanted Belter to sound to the ear—like everything and nothing at the same time—but for months as he created its syntax and alphabet, he was the only person who spoke it. To teach the producers and accent coach Armstrong the language he was creating, he had to record his voice and send it to them.

“When I first started working with him, it was like getting a recipe,” Armstrong says. “I mixed all the ingredients together and baked it. What came out unfortunately sounded too much like Jamaican.” An actor by training, Armstrong suggested some tweaks to Farmer that would make the language sound more global.

The idea was that Belter would be consistently inconsistent. accent coach Eric Armstrong

Now when you hear Rotilio and the other actors speak it, you pick up hints of familiar accents and languages—a little Italian here, some South African there, but just as soon as you think you’ve figured out which Earth-bound accent the actor is imitating, it changes. That’s by design. Belter can’t sound too much like any language on Earth.

To do that, Armstrong asked each actor to speak Belter with their own personal style. In Rotilio’s case, he used his Italian inflection. “The idea was that Belter would be consistently inconsistent,” Armstrong says. “But there are some anchor sounds that we are aiming toward regardless of how strong or weak an accent is. Like, in Belter ‘TH’ words frequently switch to a D sound, so ‘dat ting dere.’ But someone who is an extremely well-spoken Belter, we make sure they can hit those THs.” In this way, the different variations of Belter on the show are a strength, not a liability.

Giving Voice to the Belter Struggle

In the first season, Farmer translated three variations of every Belter line: one that was in pure Belter, one that was medium Belter, and one that was Belter light—basically just an accent. The showrunners didn’t want to use subtitles, so the crew had to find the right balance between an authentic-sounding language and one the audience could comprehend. To do that, Rotilio and Armstrong would work on all three versions of Diogo’s lines and shoot scenes with each of them, letting the director decide which one worked. It took time, but eventually they learned Belter light sounded the best on TV.

But Rotilio still learns the pure Belter version of all his lines. “I feel like the Belter struggle really helps give voice to a culture that has been stripped of their identity,” he says. “At the heart of it, Diogo is just a kid searching for his place in a world that doesn’t understand him. Language helps us do that.” Like Rotilio, most people watching The Expanse may be very different from Diogo—but knowing his language helps them understand where he’s coming from.