With more of us working from home than ever these days, many of us have lost access to multiple monitor setups like those we have at the office. The Duet Display app ($9.99) can help, by turning a tablet or smartphone into an external monitor. The app works well in a pinch—or even full time—and offers upgrades like wireless connectivity and Apple Pencil support for the iPad, though the latter requires paying an extra fee.

Apple's latest operating systems, Catalina on the Mac and iPadOS on the tablet, now offer built-in secondary display functionality with Sidecar, but that only works with very recent hardware models. In fact, Sidecar won't work with any iPad released before 2019. Duet Display can work with iPads dating from as far back as 2012. This makes a good case for using Duet Display in a reclamation project for an old tablet you no longer use regularly. It also works cross-platform, so you can use a Windows PC or a Mac as the host and Apple or Android mobile devices as the secondary display. And you can even turn a Chromebook into a secondary display, since Chrome OS can run Android apps.

Installing Duet

You start by grabbing Duet Display from the App Store on your iPad (or iPhone) or on Google Play. The app is compatible with iOS 10 or later and iPadOS (there's only been one version so far) and with Android 7.1 or later.

When you first run the app, it tells you to visit Duet Display's website on your PC (running Windows 7 or later) or Mac (running OS X 10.9 or later). The installation requires a reboot of the computer, which is a bit of a nuisance, and rare these days.

After downloading and installing the driver on your Mac or Windows desktop or laptop, you simply hook the mobile device up to the computer's USB port. An upgrade, Duet Air ($1.99 per month), lets you connect wirelessly using Wi-Fi and even lets you use a second desktop instead of a mobile device (though Windows 10 has a built-in Connect feature for this).

Once you connect it via the USB cable, the mobile continues to behave normally, with all sync and recharging functions active. You even see mobile notifications while using the app. When you open the Duet Display app, the computer takes control. It briefly says "Adjusting Resolution" and then you see an extended desktop on the mobile display. You can use Windows' Settings to choose whether you want the Taskbar on the second display, which isn't useful for small displays like those on smartphones. The mobile's screen can do all the typical multi-monitor functions, including extending or mirroring the desktop.

The app adds touch capability to any program you move over to that display—that's more than you get with Apple's Sidecar, which offers a non-touch display. Sidecar does, however, add a Touch bar, and drawing apps can use the Apple Pencil with it. Touch in Duet works well for scrolling and tapping icons and buttons, as well as for dragging windows around. If you usually carry your device's USB cable with you for recharging, you don't have to bring (or buy) anything extra to use Duet Display when you're on the road.

Dual Displays in Use

We used a Surface Book and several Apple and Android mobile devices to extend the desktop's interface (not at the same time: Duet only supports adding one mobile screen). Dragging a window from the computer's display to the iPad's screen works just as it would with any external monitor. That said, if you have eagle eyes, you may notice a slight lag when dragging windows around on the mobile's screen. You can minimize this delay by fiddling with the Duet Display's settings on the desktop. Setting the app's screen refresh rate from 30 frames per second (fps) to 60fps smooths out the animation and sharpens details, but it causes your laptop to work harder to maintain the higher resolution and drawing rates.

The performance options in the desktop Settings app include High Power (the best image but most battery-draining if you're using a laptop), Energy Efficient, and Pixel Perfect. The last requires Duet Pro, a $29.99-per-year add-on; Pro also adds faster drawing response, pressure and tilt control, multitouch gestures, zoom and pan, hover, line smoothing, and color calibration.

Using the iPad Air screen for social media and messaging functions is a snap. Playing 1080p HD videos using Duet Display was a smooth and stutter-free experience in our tests. Tweetdeck displayed multiple columns without a hitch, and Slack and WhatsApp windows looked fine. Using Duet for email, messaging, and social feeds while you do your main work on your main monitor makes a lot of sense.

Two Screens Are Better Than One

If you already have the latest Mac and iPad, you can use macOS Catalina's included Sidecar feature. But for people who have older devices, Android tablets, Chromebooks, or Windows laptops and desktops, Duet Display offers a simple, effective way to add a second screen to your computer—not bad for a $10 app. More ideas for using an iPad as a secondary display are in our help article, How to Use Your iPad as a Second Monitor on Any Computer.