How we check apps

iPhone Duo has a 5.4-inch outer display and a 7.6-inch inner display (1878 × 2670 pixels). Every iPhone app runs on it, but only apps built for it use the whole inner display. Fits Duo tracks which apps do.

Statuses

Fits: Fills the inner screen and adapts when you fold.
Partly fits: Some screens are still letterboxed.
Letterboxed: Runs in an iPhone-sized window with black bars.
Has issues: Works, but with bugs or broken features.
Doesn’t work: Crashes or can’t be used.
Not verified: No confirmed report yet.

Where the data comes from

Official announcements. Apps Apple shows or names in its iPhone Duo materials and developer sessions.
App Store listings. We check each app’s version and release notes daily, and a person reviews every update that mentions iPhone Duo.
Other coverage. We follow hands-on reviews, developer blogs and press articles that put apps to the test on iPhone Duo.
You. Owners rate apps and describe what they see.

Why some apps are letterboxed

Apps built with the iOS 26 SDK or earlier keep an iPhone aspect ratio on the inner display, centered with black bars, and they don’t respond to poses. Building with the iOS 27 SDK makes an app resizable. The iOS 27.1 SDK, which shipped with Xcode 27.1 in October 2026, adds full iPhone Duo support. Apple began accepting optimized apps on October 5, 2026, ahead of the October 23 launch.

If you make an app, our developer guides walk through what it takes to move up a tier.