Workplace duos aren’t a new phenomenon—in fact, they’re famously successful. Just ask Batman and Robin, Sherlock and Watson, or Mario and Luigi. However, how employees are defining these crucial relationships is changing.

In recent years, the terms ‘work wife’ or ‘work husband’ have become increasingly prevalent—so much so that a recent study found that seven in ten people in office jobs claim to have a ‘work spouse’.

A study of British office workers found that 72% of 2,000 people surveyed said they had a ‘work wife’ or ‘work husband’—a term they would use to describe someone with whom they shared a close bond.

But a so-called work spouse goes beyond the walls of the office.

Research commissioned by health and wellbeing provider Health Assured found that a fifth of respondents say they know everything about their work spouse’s personal life, and a further 23% said they’d rather confide in their work partner than their romantic partner about emotional issues.

Having a close friend at work is no bad thing—in fact it’s one of the key factors in determining an employee’s general job satisfaction.

Pew Research found last year that relationships with coworkers and bosses was the most positive aspects of many people’s roles, leading to a higher overall job fulfillment.

But experts warn that by defining these important friendships as ‘work marriages’—either publicly or between themselves—duos are signaling connotations that can spiral into conflicts and accusations.

While the quirky phrase might be shorthand between pals, it’s a red flag for everyone else.

Work wife or shared brain?

Jesse Kent and Cornelia Holvbauer have worked together for the best part of a decade.

As president and VP of New York-based PR firm Derring-Do respectively, the pair aren’t just close friends but also work so seamlessly together they’re told they “share a brain.”

While the duo would seemingly tick every box for the work spouse moniker, Kent tells Fortune he would never define the relationship so flippantly—instead sticking to the more accurate descriptors of “right hand” or “number two.”

Holvbauer was Kent’s first hire when he launched his firm nearly seven years ago and says he can rely on her as his “second brain.”

He explained: “We’re very much the same person and our clients understand that. A lot of clients ask for me because I lead the company, but I can’t be everywhere all at once. So I say to them: ‘Why don’t you chat with Cornelia because she knows everything that I’m thinking? She thinks the same way that I will.'”

The duo’s implicit trust became all the more paramount when Kent found himself traveling across the country on a monthly basis to care for a relative.

“Cornelia stepped in with all of my clients and the team to hold everything together—I didn’t even have to ask,” Kent added. “Having that personal relationship with her outside of work has made the company stronger and more successful, and me as a person stronger and more successful.”

While the relationship with Holvbauer is now so established the pair can often be “blunt” with each other, Kent said he was conscious not to alienate other team members.

“I wouldn’t be where I am without feedback from my team,” he said. One-to-one training and feedback sessions are scheduled consistently, he added: “Otherwise I wouldn’t have grown as a person or as a company.”

Know your role

Studies have often found that high-quality work relationships can result in reduced stress among staff and, as a result, greater productivity.

The benefits of close relationships with colleagues can be split into two areas, instrumental and effective, explained Dr Amanda Jones of King’s Business School in London.

Instrumental benefits are, for example, a work duo whose skills complement each other and thus are more productive as a result.

Effective benefits are more emotional factors, Dr Jones explained to Fortune, like forging strong social networks and building teams.

Not having access to these effective benefits could result in loneliness and isolation at work, Dr Jones continued, but added close colleagues needed to set clear boundaries to make sure these friendships don’t become co-dependent.

Indeed, studies have also found work friendships can get individuals through economic upset and could be a major motivator in getting staff back into the office.

Yet overly close relationships can get in the way of one’s objectivity, Dr Jones added, making people vulnerable to overworking.

Additionally: “If there’s a power differential in that relationship, it leaves people very open to accusations of abuse of power. It can lead to conflict between colleagues, accusations of kind of favoritism or even something worse than that.”

Ultimately whether these relationships become a weapon in your professional armory or an Achilles heel comes down to knowing your professional and personal roles, Dr Jones added, and not letting the two conflict.

“If you feel your relationships in your personal life are becoming damaged by the amount of attention and resource you’re giving to the other roles, that’s probably a red flag,” she added.

What to call it

Professor Celia Moore is concerned about the work spouse phrasing for several reasons, not least because it has a raft of connotations that may not be accurate.

The academic at Imperial College in London said it’s to be expected that society would coin a phrase to describe a close working partnership. After all, we do it with most of our significant relationships.

But she cautioned: “A common understanding of spouse involves exclusivity—that’s the moral imperative associated with that term. So, using the term in a workplace then has a bunch of connotations of exclusivity and special privilege … [it] can be very problematic, [and have] consequences both in work and outside of work.”

In some cases the phrase ‘work sister’ or ‘work brother’ have been proposed as an alternative for work spouse, a phrase which equally carries weighty emotional connotations.

Professor Moore said the easiest way to define these career-making relationships is to simply call them what they are: “Colleagues who have worked together for a long time” or “colleagues who manage clients together.”