It wasn’t too long ago that the exclamation mark was a point of etiquette contention in the world of email and the workplace. Style and trend pieces wondered: Are they wildly inappropriate? Are they offensive? What if you just use one?

Language is, of course, ever-evolving. So now that the exclamation mark has become mostly accepted, what’s next for English?

Emoji.

Not only have emoji arrived, they’ve arrived at legitimate non-adolescent-filled places such as messages from parents and emails among co-workers. In one survey, 76 percent of Americans said that they have used emoji in digital communication at work. Just as it is in the rest of the world, the most popular emoji in life and work is the happy face.

Lindsey Pollak, a career coach who works with Millennials, agrees that emoticons and emoji have gone from being inappropriate for the workplace to being accepted, largely because the demographic of the workplace is changing. Millennials are now the biggest generation in the American workforce, and along with them comes new technology and mannerisms.

“A few years ago, emoticons were absolutely seen as very young and very personal, and not appropriate for the workplace,” says Pollak. “Over the past few years … I've seen emoticons become more acceptable. I still have very mixed feelings about the appropriateness, but I certainly see them more frequently not just from Millennials but from all generations at the workplace.”

But why are people using emoticons or emoji in the workplace? The answer is that they’re useful. Lauren Collister, a socio-linguist at the University of Pittsburgh who studies the interaction of language and society, argues that whether it be emoticons or emoji—both are doing their part in revolutionizing language. In emails, Collister says that emoticons and emoji act as discourse particles—a word that has no semantic meaning but adds intention to a statement.

“People tend to use emoticons when there's some kind of what linguists call a face threat—something kind of awkward or potentially offensive, or somebody could take something the wrong way,” explains Collister. “So people will use emoticons or emoji in these instances to just add that little bit of extra inflection or discourse particle information at work too because it's a useful way to communicate.”

It’s for this reason that the happy-face emoji dominates.  As Will Schwalbe, co-author with David Shipley of the classic email etiquette book Send: Why People Email So Badly and How to Do It Better (sometimes regarded as the “Strunk and White of email”), explains, “The biggest problem about all electronic communication is that it's toneless. In the absence of tone, people read negative tone into it.”

“Whether you're using the exclamation mark, which we called the ‘ur emoticon’, or emoticons, or emoji, they all serve the same incredibly valuable purpose which is they take this very dull, flat, affectless form of communication and they make it cheerful, friendly, they bring a smile … They kick it up a notch,” says Schwalbe.

A Scandinavian study on email in the workplace found exactly that: Emoticons in the workplace were not used to convey emotion, but rather to signal how the information in the email should be interpreted. They found three primary uses: to express positive vibes, to mark jokes, and lastly to either strengthen or soften statements that could be misread as reprimanding. An American study found that on that last point, smiley faces in email can reduce negative interpretations.

Along with the usefulness of emoticons and emoji in clarifying tone in emails, another partial explanation for the rise of emoji at work is that digital work communication now incorporates casual communication as well. “In the past email was seen as more formal just in general—because it's written and it's kind of like a letter,” says Collister. “But I think that's changing with how we use email for everything these days.”

Beyond email, the growing popularity of office collaboration and communication tools like Slack are increasingly taking casual work interactions online. “Casual communication is a perfectly valid type of office communication that's always existed,” says Schwalbe. In the past, these interactions—whether it’s to tell a joke or ask someone how their weekend was—were reserved for in-person or on the phone. Nowadays, there’s a Slack channel for that—whether it’s Game of Thrones fans or baby photos. This moving of casual office communication online—and therefore into text—has contributed to people’s comfort level of using emoticons and emoji with their co-workers.

Stewart Butterfield, the CEO and co-founder of Slack, says that one of the aims of the tool was let people feel comfortable with these casual interactions online. "One of our aims for Slack is to help people 'bring their whole selves to work',” says Butterfield. “That might sound a little lofty, but we believe there is a widespread feeling that people are meant to check a lot of stuff at the door when they arrive at work. Some of that makes sense, but there's a risk of having people feel diminished or unable to contribute fully—that's the part we hope Slack can have a shot at correcting."

Pollak, however, warns against being too casual at work. Her advice is be conscious of who the audience is, and gauge their comfort level before putting in that emoji. “Frankly, I wouldn't use a smiley face with any CEO in America. I wouldn't use a smiley face with a certain level of executive no matter how commonplace and acceptable they've become,” says Pollak. “You can make or break a relationship with one email these days, so you have to be really careful.”

In other words, the usual office rules still apply with emoticons and emoji. For example, don’t use them with a superior or a client unless they use it first and establish it as an accepted norm. And never use the eggplant emoji at work.

Linguists call this register. Register is the idea that there's different kinds of language that we use in different situations. I would never talk the same way to my boss the way I would talk to my friends at the bar after work,” says Collister.

In that sense, even as people’s comfort level with emoticons and emoji in the workplace rises, communications with co-workers online shouldn’t deviate too far from good colleague behavior offline. Schwalbe’s advice is just to use common sense.

“No one likes sarcasm. It's a very bad form of work communication. Using emoji or emoticons in a sarcastic way is just as bad as it's always been,” says Schwalbe. “We should bear in mind that sarcasm existed long before emoji, and it's always been a bad tactic. We didn't need emoji to be obnoxious.”