73 Duo projects show how hinge APIs remake phone games

iPhone Duo hinge apps treat the fold itself as a button, a dial or a story beat. Apple gave developers new hinge APIs (software hooks that report data from the hardware), and they tell an app two things: how far open the phone is, and how quickly you are moving it.

That sounds small, but it changes what an app can be. A normal app on a bigger screen just gets more room. An app that reads the hinge can react to a physical gesture, the same way a game reacts to you tilting the phone. Opening, closing, snapping and half-folding all become inputs.

The Duo also has an unusual screen setup that helps. Its inner display is 7.6 inches and its outer display is 5.4 inches, and Apple pointed out at its September 9 launch event that the two share the same shape. A developer can design one layout and have it work on both.

The burst of ideas came from a September 26 hackathon run by Bitrig, a company that makes an app-building tool. Entrants built 73 projects designed only for the Duo, from interactive storybooks to instruments to games that are hard to describe.

The winner was Crate, a music sampler. Playable pads sit on the bottom half of the screen while the top half shows controls. Folding the phone changes the audio effects, and snapping it open can trigger a drop in the track.

Another standout was Accorduon, an open-source app by developer Artem Novichkov. Bass notes sit on the left half and piano keys on the right, and moving the hinge works like the bellows of a real accordion. Pump it faster and the sound gets louder and brighter.

Then there was duo-lingo, a flashcard prototype. The question shows on the outer screen and the answer slowly appears as you open the phone. Its developer, Aryan Keluskar, says he built it in about four hours with help from Bitrig’s Claude Code integration, an AI coding assistant. Another entry, Arden, lets you describe a game in plain words and then generates a playable version with fold-friendly controls.

Several of these ideas have grown into paid or free apps since launch. A few worth knowing:

• Toastie ($2.99). You build a sandwich to a recipe on the inner screen, then close the phone for the stated number of seconds to grill it. Open it too late and it’s burned. There are only 10 recipes, so it’s a short, silly treat.
• Deck-Man ($4.99). A retro cassette player that uses your Apple Music library. You pick an album, insert the tape and shut the phone to press play, complete with mechanical clicks. Before launch, developer Vidit Bhargava showed a very similar Walkman-style app called Duo-Man.
• PianoFold (free to start, $5 to unlock). A rhythm game played with the phone bent to a right angle. Blocks fall toward the crease and you tap in time. It has 40 levels in eight chapters, and the first two chapters are free.
• Delta (free). The long-running game emulator now turns a half-folded Duo into a dual-screen handheld that looks like a Nintendo DS. Closing the phone even puts the emulated console to sleep.
• RectoVerso ($13). A grown-up version of the hackathon flashcard idea. It builds cards from your notes or a PDF using Apple Intelligence, and you open the phone a little to peek at the answer.

Bigger names have joined in too. Netflix moves its playback controls to the lower half when the phone sits like a tiny laptop, and the flight tracker Flighty shows a map beside your flight details when the Duo is open. Look for a “Made for iPhone Duo” badge in the App Store, which Apple puts on apps that use the larger screens well.

If you bought a Duo, expect a mixed first month. The fun hinge apps are here, but plenty of everyday apps still look awkward, with odd gaps and stretched layouts on the new screen shapes.

That’s normal for a brand-new form factor. The first Samsung Galaxy Fold in 2019 had the same problem, and so did the iPhone X in 2017, when apps that hadn’t been updated showed black bars. Android foldables have had about seven years for developers to catch up. The Duo’s single design and matching screen shapes should make updates quicker, but Apple can’t set the pace for every company. It took Instagram 15 years to ship a proper iPad app.

Also keep the price in mind. The Duo costs about $2,000, so these apps are a bonus, not a reason to buy one on their own. Android foldables have run similar fold-aware experiments for years. What’s new is the sheer number of indie developers trying ideas on a single, popular device.

More are on the way. Twofold, a set of tabletop games that uses the raised half as a privacy screen so a rival can’t see your pieces, already had an App Store listing ahead of launch. Indie veteran James Thomson has been testing folding layouts for his PCalc calculator.

Apple has also said stylus support will come to the Duo later this year. When it arrives, planner and notes apps such as FirstSeed Planner, which already puts a calendar on one half and a daily plan on the other, should feel much more like paper.

Not every experiment will last. Some are clearly one-joke apps you’ll open twice. But AI coding tools now let a single developer test an idea in an afternoon, so the Duo’s App Store section is likely to keep getting weirder for a while.