Got an iPad and this app? You’ve got a second Mac screen

Ever since the iPad first came out, people have wanted to use their Apple tablets as a second computer monitor, but most of the software that makes that possible transfers data over Wi-Fi — introducing lag and inconsistency. A new app called Duet Display will turn your iPad into an external screen for your Mac, and it does it over the cable you already use to charge and sync.

Duet Display is now available on the iOS App Store. For now, it costs $9.99, but at some time in the future that price that will bump up to $14.99.

Duet Display is developed by Rahul Dewan, who was an engineer at Apple before starting Duet Display. It’s actually two apps — one for your iOS device and a companion app for your Mac that lets it recognize an iPad or iPhone as a second screen. To get Duet Display working, simply open the Duet Display app on your iPad and plug it into a modern Mac with a Lightning cable (or a 30-pin cable if you’ve got an older iPad that’s still supported.) Because you’re plugged in, there’s no need to worry about killing your iPad’s battery.

The Duet Display Mac app puts a little settings icon on the right side of your menu bar. Duet Display requires you to plug in your password when you’re installing it on your Mac because the app needs to install display drivers. As of now, you can only connect a single iOS device, unfortunately.

Duet Display set-up screenshot. It's this simple.

Duet Display set-up screenshot. It’s easy to get up and running.

This is weird: While you’re using Duet Display on the iPad, you’ll still get your regular iOS notifications. It’s only an app, after all. You can also use your iPad’s touchscreen, although sometimes it’s a little wonky because OS X isn’t optimized for touch.

Although Duet Display said its implementation has “zero lag” at 60 frames per second, I noticed my mouse straggling a little bit on the iPad. Although Duet Display isn’t lag or bug-free, the experience is a good deal smoother than the Wi-Fi based implementations I’ve tried in the past, including Air Display. Your iPad comes up as a fully fledged 2nd monitor in the OS X displays setting menu, and you can set your computer to use all the pixels on your iPad’s Retina display, or you can set your iPad to “regular” resolution as well.

A Mac running Safari displaying TweetDeck on an iPad Mini with Retina Display through Duet Display

A Mac running Safari displaying TweetDeck on an iPad Mini 2 through Duet Display

Duet Display works with all Macs running OS X Yosemite, and any iPhones or iPads running iOS 5.1.1 or up — so it’s a great reason to dust off an old iPad.

This story was updated on 12/18 to add additional information on iOS App Store availability and a link