The short answer
iPhone Duo has an outer display for when it is closed and a larger inner display for when it is open. Apps built with the iOS 27.1 SDK or later are optimised for it; older builds run, but do not adapt fully. The apps that break are the ones that size themselves from the screen, make layout decisions from orientation, or assume that a wide layout means iPad. And from April 2027, App Store submissions need iPhone Duo screenshots.
What is different about iPhone Duo
From Apple’s design guidance for iPhone Duo:
- Two displays, each with its own front camera. People use the outer display when the device is closed and the inner display when it is open.
- Controls on the side. The outer display is wider and shorter than other iPhones, so the system moves toolbars and tab bars to the side to keep vertical space for content. They stay on the side on the inner display in landscape; only the inner display in portrait keeps horizontal bars.
- Reserved regions. The outer camera is always present and expands into the Dynamic Island for Live Activities. The inner camera only appears when it is active. When the device is partly open, the fold divides the inner display.
- Poses. People hold it partly folded like a book, lay it flat or stand it on an edge.
The SDK you build with decides how your app looks
Apple describes three outcomes, depending on the SDK in your build:
| Built with | On iPhone Duo |
|---|---|
| iOS 26 SDK or earlier | Runs, but does not adapt. On the inner display the app sits in the centre with empty space around it; on the outer display its content sits beside the status bar and camera. |
| iOS 27 SDK | Resizes to fill most of the inner display, but still avoids the status bar on the right edge. |
| iOS 27.1 SDK or later | Optimised: the app uses the full display, with toolbars and tab bars placed vertically below the status bar. |
Xcode 27.1 adds development support for iPhone Duo. Before you ship a build with the 27.1 SDK, Apple’s advice is to check that your content adapts to bars that become vertical.
The code patterns that break
Apple’s preparation checklist names the usual culprits. Search your project for each one:
- Sizing from the main screen.
UIScreen.main.boundsdoes not match your app’s size on iPhone Duo, and the size changes when the device folds. Size from the view or window instead (view.bounds,window.bounds, orGeometryReaderandonGeometryChangein SwiftUI), and read it again whenever it changes, not once at launch. - Orientation as a layout signal.
UIDevice.current.orientation,statusBarOrientationandinterfaceOrientationdo not tell you what shape your app is. Compare the width and height of the space you actually have. - Idiom or size class as a device check. On iPhone Duo an iPhone app can have any combination of size classes. Code that shows an iPad layout whenever the width is regular will now run on iPhone. Remove
userInterfaceIdiomchecks from layout code, and any comparison with a known screen height. - Screens and windows. Read the display scale from the trait collection (
traitCollection.displayScale, or@Environment(\.displayScale)in SwiftUI), create windows withUIWindow(windowScene:), and do not keep aUIScreenaround. - Full-bleed media. Images and video set to fill can lose their subject on the wider inner display. Choose between fill and fit from the space available, or set a focal point.
Design: let the layout grow, do not redesign it
Apple does not ask for a custom layout for every pose. A compact width layout for the outer display and a regular width layout for the inner display cover them all, provided the layout is built from size classes, margins and safe areas. Keep functions and state the same on both displays, and show an extra level of hierarchy on the inner display where it helps: Mail shows a list or a message when closed, and both side by side when open.
- Keep the system’s vertical bars. Standard toolbars and tab bars move to the side automatically; Apple advises against overriding that placement.
- Give every toolbar item a title and a symbol, and set visibility priorities so the important actions stay visible when space runs out.
- Respect the fold. Prefer containers that adapt, such as split views, use an even number of grid columns, and use the ReservedRegion APIs to keep important elements clear of the centre.
- Change little while folding. Small adjustments are easier to follow than a rearranged screen.
- Consider arrangement views. New containers,
ArrangementViewin SwiftUI andUIArrangementViewControllerin UIKit, place two views side by side or on top of each other depending on the space and the fold.
How to test without the device
- The iPhone Duo simulator in Xcode 27.1. Apple calls it the best way to test the full native experience.
- The resizable simulator in Device Hub, to see your layout at sizes between the two displays.
- iPhone Mirroring on macOS 27, with the window dragged to extreme sizes in both directions.
- Xcode’s resizability skill. Ask the coding assistant to get your app ready for iPhone Duo and it scans for the patterns above. Apple notes that it finds most issues, not all of them.
The App Store side
- Screenshots: 1398 × 2034 pixels for the outer display and 2007 × 2853 for the inner display, or the same turned sideways. Apple’s specification makes them required from April 2027 for apps using the iOS 27.1 SDK or later. Check yours with our screenshot size checker.
- A badge: when you submit iPhone Duo screenshots, your product page shows that the app is optimised for iPhone Duo.
- A preview: the product page preview tool in App Store Connect shows your screenshots and creative assets on iPhone Duo before you submit.
- Featuring: you can mention iPhone Duo support, including all device poses, in a featuring nomination.
Checklist
- No
UIScreen.mainin layout code; sizes read from the view or window, every time they change - No layout decisions from orientation or device idiom
- Layouts built on size classes, margins and safe areas, tested at both widths
- Standard toolbars and tab bars, with a title and a symbol on every item
- Nothing important under the fold or the cameras
- Full-bleed images that keep their subject on the inner display
- Tested in the iPhone Duo simulator, then built with the iOS 27.1 SDK
- iPhone Duo screenshots ready before April 2027
Sources: Apple’s Get ready for iPhone Duo, Three steps to make your app shine on iPhone Duo, Designing for iPhone Duo and Prepare and submit your apps for iPhone Duo.
Related: App Store screenshot sizes · What’s new in iOS 27 · SwiftUI vs UIKit