Video and photography enthusiasts have argued over the use of vertical video, which looks natural when holding a mobile screen upright, but offers a very narrow field of view and doesn’t make full use of computer monitors and televisions. Not long ago, Snapchat founder Evan Spiegel pitched the idea of mobile advertising specifically geared for the vertical format, and other companies have been dabbling with it too. Now, it appears that it has finally taken off, and millennials could be following suit as a result.

Forbes, using numbers provided by Mary Meeker’s 2015 Internet Trends report, writes that vertical viewing has increased dramatically over the past five years among millennials, going from a lowly five percent in 2010 to 29 percent in 2015. A number of social media-based formats are starting to use it more regularly, including Snapchat and Twitter’s live-streaming Periscope.

ResizedImage600370 Chart

The chart above breaks down the time spent between horizontal viewing and vertical viewing across a number of devices, including mobile, desktop/laptop and television. As you can see, while horizontal screens still have a significant lead, vertical viewing has been catching up.

With popularity picking up across both formats, Snapchat has specifically tried to work on an advertising program that appeals more to the vertical demographic, as noted in Spiegel’s pitch. Since it has an audience of approximately 60 percent millennials, it’s easy to see how the format would appeal to them, with “vertical ads for vertical screens,” as the company has pitched to marketers

Snapchat also notes that vertical video ads have a completion rate that’s 9X better compared to horizontal mobile video, due to better interaction of ads and the way they format onto the display. They seem less cramped, even if they are narrower.

“More than ever before, marketers must focus on ways to engage millennials and ultimately differentiate themselves among the crowded digital space — in this case, how to present and optimize their digital content for vertical screens,” reads the article. “A slight adjustment in screen orientation for digital content can go a long way to increase engagement with millennials. Brands can no longer ignore vertical content and must begin integrating this new approach into their existing content strategy as a way to delight and engage millennials.”