What Exactly Is a Quaternary Marriage? 'Extreme Sisters' Fans Are Curious

Season 1 of Extreme Sisters casts light on the ups and downs of having an identical twin.

Featuring stars like Anna and Lucy DeCinque or Brittany and Briana Salyers (née Deane), Extreme Sisters celebrates the unique bond some siblings share.

Being a twin is a crucial part of the stars' identity. This is particularly true for Brittany and Briana, who opted for a quaternary marriage. So, what does a quaternary marriage entail?

Season 1 of Extreme Sisters puts twin sisters like Anna and Lucy into the spotlight.

Anna and Lucy describe themselves as "the world's most identical twins," and they prefer to think of themselves as one person in two bodies. What's more, being a twin directly influences their dating habits — as Anna and Lucy have been with the same person, Ben Byrne, since 2012. But so, is polyamory any different to quaternary marriages?

While Anna and Lucy insist on dating the same person, Brittany and Briana opted for a slightly different approach. The stars of Extreme Sisters have always dreamed of marrying twin brothers, and the ambition became a reality in 2018.

They first met Josh and Jeremy Salyers at the Twins Days Festival held in Twinsburg, Ohio, in August 2017. Their wedding ceremony officiated by twin priests and featuring twin ring bearers was held at the same festival a year later. But quaternary marriage isn't just about a joint wedding.

After a joint honeymoon to the Canadian province of Prince Edward Island, the foursome moved into a family home in Virginia. As Jeremy told Fox News, they have been asked many questions about their living situation over the years.

Instead of two separate households, Brittany, Josh, Briana, and Jeremy consider themselves to be one family unit. As Jeremy told the outlet, they frequently respond to queries about their lifestyle — but most of the feedback they get is positive.

"We get a lot of questions about our living situation, [we get] mostly positive responses to when people find out about it," Jeremy said. "We don't mind talking about it at all."

"I would add that we totally know we are eccentric, socially eccentric," Briana said. "I would say that the one that we get a lot [is that] people always ask if we switch partners — I hope we have explained ourselves that we do not!"

According to The New York Times, a quaternary marriage mostly refers to the special emotional and socio-economic bond two sets of twins have.

A quaternary marriage often sees husband-wife duos share responsibilities such as child rearing or domestic chores. Couples living this way are likely to support each other in multiple areas of life.

The dynamic between Brittany, Josh, Briana, and Jeremy can easily be compared to what Robert describes in the 1972 text. They live under the same roof and they are raising their baby boys, Jett and Jax, together.

"When we have children, my and Josh's children will be genetic siblings to Briana and Jeremy's children," Brittany told People in 2018, before she and Briana welcomed Jett and Jax. "Even though they're cousins, they're technically genetic siblings. We imagine it will be like two moms and two dads all raising our families together."

Could they be onto something? There's only one way to find out. Catch new episodes of Extreme Sisters every Sunday at 10 p.m. EST on TLC.