A new kind of iPhone
I'm building apps for iPhone Duo, and writing down everything I learn on the way. This is where that lives: what I'm making, a reference for developers, a directory of Duo apps, and notes from the work itself.
The same device is a phone, a small tablet, a book and a tiny laptop, depending on how far it's open and how it's standing.
A set of apps where the fold is the reason for the work, not a layout the app tolerates. I name each one when it is close to shipping.
Comics read like a book: the fold is the gutter, and each page gets its own leaf.
A teleprompter that records you. The script faces you on the outer screen, right beside the lens, and follows your voice.
Games for two people and one phone, played across the table and with the hinge.
Books and paint that behave like paper when you open and close them.
Instruments where folding the phone is part of how you play.
Rebuilding with the current SDK gives every app the bigger screen for free, so that alone is not a reason to choose one app over another. These are the parts only this phone has.
Outer screen while the camera runs
While an app is using the camera, full screen on the inner display, the system can show content from the app on the outer screen, facing the person being filmed, including simple controls. Apple says camera use is the only supported case, for now.
CameraCaptureAccessoryUISceneAccessory.cameraCapture A live hinge angle and state
Apps can read whether the device is closed, partly open or fully open, plus a continuous angle. Apple positions it for interactions and effects rather than layout, and makes no promise about how often it updates.
onHingeChangeUIHingeInteraction Book, tabletop and flat
Partly folded, the fold becomes a reserved region that splits the inner screen. That gives a book with a spine, or a tabletop with a view above and controls below.
reservedRegions(kind: .division)ArrangementView
Regular width on the inner screen
The inner screen reports regular width, like an iPad. Every app rebuilt with the current SDK gets this, so on its own it does not make an app a Duo app.
size classesScreen sizes in points, the hinge and fold APIs, the outer screen rule, cameras, SDK behaviour and the simulator, checked against the SDK and kept up to date.
Open the reference → DirectoryApps for the Duo, with what each one does on it and where that claim comes from. Mine first, and good ones from other developers as they appear.
See the directory →Technical notes and the reasoning behind what I build.
Fold Cue's best feature on iPhone Duo is the script on the outer screen, beside the camera. The system decides when that screen is mine, and I won't hold a Duo until it goes on sale. So the app is designed to be fine without it.
Fold Cue was planned in a hurry, for a phone that isn't out yet. The most useful part of the plan wasn't the feature list. It was deciding in advance what would never be traded away when time ran short, and what the app wasn't going to try to be.
iPhone Duo isn't in anyone's hands yet, and I'm already building for it. Why now, what makes an app a Duo app rather than a big phone app, and the kinds of app I've decided not to build.
Running ComicFlow in the iPhone Duo simulator showed stretched layouts, phone-sized covers and a reader that decided on spreads by orientation. The Duo exposed them, but several of the bugs were already shipping on iPad.
The simulator, App Store Connect and Apple's spec sheet give different pixel sizes for the iPhone Duo's inner display. None of them is wrong. The simplest explanation is that the system renders larger than the panel and scales down, which is one more reason to lay out in points.
Third-party apps can show interactive content on the iPhone Duo's outer screen while they run on the inner one, but only through a camera capture accessory. That one rule decides which Duo ideas are possible, and it closed off a few I wanted to build.
iOS 27.1 gives every app the iPhone Duo's hinge status and a live angle, with no entitlement. The API is small. The interesting part is the one promise Apple leaves out, and how to design an interaction that doesn't need it.
The iPhone Duo simulator ships in the Xcode 27.1 beta. Getting it running took one surprise from xip, a renamed Simulator app, a permission error on my external drive, a noisy first boot, and screenshots that came out black.
The questions people ask most about iPhone Duo, answered from Apple's own pages and the SDK.
iPhone Duo is a trademark of Apple Inc. This site is independent and is not affiliated with, sponsored by, or endorsed by Apple. Facts last checked September 26, 2026. Spotted something out of date? Tell me.