Email, text messaging, and chat apps might seem the perfect tools for deaf people to communicate. But those with little or no hearing are a visual bunch, and many prefer sign language, says Claude Stout, executive director of TDI, an organization that promotes equal-access technology for the deaf and hard of hearing. “We show the nuances of communication,” says Stout, through a sign interpreter. “And we use our expressions to show our feelings, and show that we are happy or sad or concerned or upset, just like you can hear those nuances in a person’s voice.” And signers can talk fast, says Stout, at up to 200 words per minute. Furthermore, signing is often the native language for those who use it. Moving to the keyboard means switching to a second language.

That’s why Stout and his colleagues at TDI were excited to find Glide, an Israeli startup founded in 2012 that makes a free video-chat app of the same name for Android and iOS. Glide told me that it now has “at least several hundred thousand deaf users.” (The app has been installed on more than 20 million devices and Glide claims “millions” of active users.) “They were a community that we found accidentally,” says Sarah Snow, Glide’s community manager. Snow was making YouTube videos about Glide when she started getting comments from app users asking her to add subtitles. “When I first saw those messages, I didn’t know what to think; I didn’t know how many deaf users we had,” she says. “But I knew that they were a community that always responds to my videos.”

Finding a trove of unexpected fans, Snow went all-out to cultivate them. That meant not only adding captions to her YouTube clips, but starting to learn American Sign Language (ASL) so she could make videos specifically for this community. Snow has also done meetups for users and institutions for the deaf and hard of hearing, including Gallaudet University in Washington, D.C., and the schools for the deaf in Austin, Chicago, New York City, and Fremont, California. Glide users even proposed their own sign for the app, with the most popular being a fist with thumb and index finger extended out. Snow describes Glide’s relationship with its deaf users in a video promoting a South by Southwest panel on the experience she’s presenting in March 2016.

Sarah Snow flashing the sign for Glide that the app’s users created.

Glide isn’t the first tech company to discover fans it never anticipated. Igniter.com, for instance, was founded in 2008 as a group-dating service focused on New York City, but it soon became the fastest-growing dating site in India. “In January 2010, we made the decision that we are an Indian dating site,” Igniter cofounder Adam Sachs told the New York Times in an interview.

Glide is far from the first video-chat service: Skype was founded a dozen years ago, and FaceTime debuted on the iPhone 4 in 2010. And of course Snapchat has video. But Glide has one killer feature for deaf people: the ability to leave a video message rather than having to prearrange a live call. “With Glide, they can send a message whenever they want,” says Snow. “They don’t have to wait for someone to answer a call.” With that asynchronous messaging capability, sign language users get the same flexibility everyone else has with tools such as email or Facebook messages.

Glide has one killer feature for deaf people: the ability to leave a video message.

The app could do more, though, Snow soon learned. Glide uses a feature called optimize video frame rate, in which it skips some video frames when bandwidth is limited. A hearing person might appreciate trading quality for speed, but dropped frames could garble sign communication. At the request of deaf users, Glide added a setting to turn off the feature. “If you send a video and you have a poor connection, then it might take a second or two longer to send,” says Snow, “but it will play smoothly.”

In August 2015, TDI chose Glide for its biennial Andrew Saks Engineering Award for enabling technologies. “We wanted to show the world that Glide’s video technology is starting out as a mainstream technology for anyone who wants to use it,” says Stout. “But Glide [is] proceeding with advertising and promoting this technology as a benefit in the deaf community.” The previous two recipients of the award were Microsoft in 2013 for its overall commitment to accessibility, and Google in 2011 for its technology to automatically create captions on YouTube videos. (The feature was introduced in November 2009.)