The iPhone Duo Is More Interesting in the Details Than the Fold

Apple certainly did not invent the foldable phone. Royole shipped the FlexPai in 2018, Samsung followed with the Galaxy Fold in 2019, and Samsung has now had eight generations to work through many of the problems that come with folding a display in half. Huawei, Google and others have been building foldables for years too. Apple is arriving very late to this particular party.

I don't really care.

I have watched foldable phones become increasingly interesting, but what I wanted was one built around the ecosystem I already use every day. Apple has had the advantage of watching everyone else figure out hinges, creases, flexible display materials, awkward aspect ratios, battery placement, cameras and software for nearly a decade before finally deciding it was ready to ship one.

And this thing has been rumored for almost that long. Reports of Apple investigating folding displays go back to at least 2016. There were reports involving LG in 2017, Samsung display samples in 2019 and 2020, early internal work reported in 2021, and even reports that Apple experimented with clamshell-style prototypes before settling on the book-style design that became the Duo.

After years of "foldable iPhone coming next year" stories, it is strange to finally have an actual product page.

I am also probably more sentimental about Apple hardware than most people. I still have my original iPhone, original iPad, original Apple Watch and original iPad Pro. I still have the 17-inch MacBook Pro I bought when I was finally able to afford Apple hardware. I don't keep every device I have ever owned, but I have kept the ones that marked a meaningful change in how I used computers.

The Duo feels like it might be another one of those.

Folding it is only the beginning of the engineering problem

The obvious specification is the screen. Closed, the Duo has a 5.4-inch outer display that Apple says provides more than 90% of the screen area of an iPhone 18 Pro. Open it and you get a 7.6-inch display with 50% more screen area than the iPhone 18 Pro Max.

What interests me more is everything Apple had to put behind that screen to make it survive being folded thousands of times.

According to Apple's hardware documentation, the inner display is made from 10 ultrathin layers. The surface has a scratch-resistant coating and a nano-textured cover made from a custom polymer that Apple says is up to 40% stiffer than comparable materials used elsewhere in the industry. There is high-strength glass both above and below the folding OLED panel, followed by a titanium plate underneath the entire assembly.

The adhesives between those layers are interesting too. Apple says they are designed to allow the individual layers to move past one another like pages in a book as the phone folds. That relieves the stress that would otherwise build up as several layers with slightly different bend radii are repeatedly folded together.

That is the sort of detail I love.

A foldable phone is not really one engineering problem. It is dozens of mechanical problems hiding behind a screen. Glass doesn't particularly enjoy bending. Batteries don't bend. Circuit boards don't bend. Camera modules don't bend. Antennas need space. Heat still has to go somewhere. Then you have to connect two halves of a computer through a moving mechanical joint that somebody is going to open and close all day for years.

Apple's hinge contains more than 100 components. It supports the center of the display when the phone is completely open so the screen stays flat, while magnets provide the final closure when it is folded. The frame and hinge cover are grade 5 titanium, and Apple is actually 3D printing the titanium hinge cover before giving it a micro-blasted finish.

Internally there are structural ribs and even ceramic-fiber inserts around antenna splits to increase rigidity.

Those details are considerably more interesting to me than another annual CPU benchmark.

Despite all those moving pieces, Apple rates the Duo IP68 for water and dust resistance. I am going to be very interested in the eventual teardowns because getting a folding mechanical joint, two displays and all of those internal moving relationships into something with an IP68 rating is not trivial.

There really is a camera underneath the display

One of the hardware details I find most fascinating is almost hidden in Apple's announcement.

When the Duo is open, there is no normal camera cutout interrupting the large inner display. Instead, Apple put a FaceTime camera underneath the display itself.

The camera lens is physically behind the folding OLED, so it has to collect its image through the display area sitting above it. Under-display cameras are not entirely new to the phone industry either, but this is the first time Apple has shipped one, and Apple has been surprisingly quiet about exactly how it works.

I went looking through the technical specifications expecting to find considerably more detail.

There isn't much.

Apple lists the inner camera as an "Under-display FaceTime camera" with an ƒ/1.8 aperture and 1080p video at either 30 or 60 frames per second. Apple does not even publish a megapixel count for it.

That becomes particularly interesting when you compare it with the camera on the outer display. The outer camera is a 12MP Center Stage camera capable of recording 4K Dolby Vision video at up to 60 frames per second.

I don't think that difference is accidental.

The spec sheet suggests that Apple considers the hidden inner camera primarily a FaceTime and video-conferencing camera where having an uninterrupted 7.6-inch display is more valuable than maximizing image quality. I am making an inference there because Apple has not said that explicitly, but the specifications certainly point in that direction.

I would really like to know what Apple is doing at the display level above that camera. Under-display cameras have historically had to deal with less light, diffraction and the fact that there are literally display components between the subject and the image sensor. Apple has not published enough information yet to know how it is handling those tradeoffs.

That may have to wait for someone to put one under a microscope.

What I like about the rest of the camera design is that Apple doesn't really need the hidden camera to be its best selfie camera anyway.

Two displays make the rear cameras much more useful

This may end up being one of my favorite things about the Duo.

The rear camera system has a 48MP Fusion Main camera and a 48MP Fusion Ultra Wide. There is no dedicated telephoto camera like the iPhone 18 Pro gets, which is one of the real compromises Apple made to fit everything into this design. The Main sensor provides an optical-quality 2x crop, so the available focal lengths are effectively 0.5x, 1x and 2x.

But because there is a display on the other side of the phone, the rear cameras can now become selfie cameras.

Turn the phone around and the outer display can show you what those 48MP rear cameras see. That gives you a much better camera while still letting you frame yourself normally.

Apple calls another implementation of this Duo Preview. If I am taking a picture of somebody else, they can see the live composition on the outer display while I see the camera controls on my side. They can adjust their pose or framing themselves rather than having me take five photographs and then hand over the phone to see whether they like any of them.

That feels like one of those small uses for a second display that may prove much more useful than it sounds in a keynote.

Smart Take goes further. The A20 Pro analyzes the scene locally and can recognize when everyone is posed, then take the photograph automatically. Instead of setting a timer and running back into the photograph, I can pose the Duo, get into the picture and let the phone decide when everyone is ready.

As a dad, I also laughed at Kid Cue. Apple uses the outer display to show Peanuts animations near the camera so kids will actually look toward the lens.

Some engineer involved in that feature has definitely tried to take family photographs.

The fold becomes useful for video too because the phone can stand by itself. I can partially fold it, put it on a table and use it for FaceTime, time-lapse photography or video without carrying a tripod or leaning my phone against whatever random object happens to be nearby. Duo FaceTime can even use the outer display to let somebody sitting across from me participate in the same video call.

This is the kind of thing I hoped Apple would do with a foldable. The second display and hinge aren't just there so the specification sheet can say the phone becomes larger. Apple is finding places where the physical shape creates interactions that weren't possible on a normal iPhone.

Apple Pencil on an iPhone is a much bigger change than it sounds

The other feature I am particularly excited about is Apple Pencil support.

Later this year, Apple says the Duo will work with Apple Pencil (USB-C) on either the inner or outer display. I plan to get one for mine because a 7.6-inch screen I can carry in my pocket suddenly becomes a very convenient place for handwritten notes, quick diagrams, markup and sketches.

This is another place where reading the fine print matters.

Apple has only announced support for the USB-C Apple Pencil. It has not announced compatibility with Apple Pencil Pro. The USB-C Pencil has pixel-level precision, low latency, tilt sensitivity and hover, but it does not have pressure sensitivity, double-tap, squeeze, barrel roll, haptic feedback or wireless pairing and charging.

I am a little disappointed by that.

For the way I expect to use it, mostly writing, diagramming and marking things up, the USB-C Pencil should be fine. But if Apple is going to turn an iPhone into a small folding notebook, I would eventually love to see full Pencil Pro support. Pressure sensitivity in particular would make the Duo much more interesting as a genuinely portable drawing device.

I would specifically buy the USB-C model only after Apple enables Duo support. Apple currently names that model and not Pencil Pro, and its own comparison confirms the USB-C Pencil lacks pressure sensitivity.

Still, Apple Pencil support on an iPhone is something I have wanted for a long time. Folded up, it is still a phone. Open it and suddenly I have something approaching a small iPad that I can write directly on.

That makes much more sense to me than carrying an ever-larger slab-shaped phone.

The packaging inside may be the most impressive part

The Duo is only 5.2 millimeters thick when open. Each half has to contain enough of a computer to leave room for the hinge, displays, cameras, cooling, radios and batteries while still closing into an 11.3-millimeter-thick phone.

It weighs 254 grams, so nobody should confuse this with an especially light phone. There is a lot of hardware in there.

Apple put one high-energy battery in each half and uses software to balance them so they behave like one power system. Apple rates the Duo for up to 24 hours of typical mixed use, 31 hours of video playback using the inner display and 44 hours using the outer display.

Even the decision to make the Duo eSIM-only worldwide was partly a mechanical engineering decision. Apple explicitly says eliminating the physical SIM hardware freed internal space so it could put more battery into the phone.

That is the kind of compromise that gets lost when specifications are discussed individually. On something this thin, every cubic millimeter is competing with something else.

The A20 Pro creates another packaging problem because a large display invites things like gaming, multitasking and heavier applications that generate heat. Apple created a new chip package inspired by its M-series designs that connects the A20 Pro directly to a custom vapor chamber. Apple claims that arrangement gives the Duo up to 35% better sustained performance than the iPhone 17 Pro.

There is also a new display engine specifically responsible for driving the inner and outer screens simultaneously and transitioning between them. The A20 Pro itself is built on a 2-nanometer process with a six-core CPU, seven-core GPU, dual 16-core Neural Engine and 50% more unified memory bandwidth than the A19 Pro.

Then there are two more Apple-designed chips that don't get much attention in the headline specifications. N1 handles Wi-Fi 7, Bluetooth 6 and Thread, while Apple's C2 cellular modem handles the cellular side and supports mmWave in the United States.

Apple is increasingly turning the iPhone into a collection of its own silicon rather than something assembled around a single Apple-designed application processor.

That is interesting by itself.

Touch ID is back for a reason

Another quiet hardware decision is that the Duo uses Touch ID in the side button.

There is no Face ID system listed in Apple's specifications.

Apple says putting Touch ID in the button gives the phone consistent authentication whether it is open or closed. That makes sense when the same device can be held in several orientations and either display might be facing me.

I have always liked Face ID, but I don't think moving back to Touch ID on this particular device is necessarily a regression. A fingerprint sensor on the button is available regardless of which screen I am looking at, how the phone is folded or whether I have it sitting on a desk.

Apple Watch can also unlock the Duo.

Again, the interesting part is how many apparently small design decisions have to change once "the front of the phone" stops being one fixed thing.

iOS may be where Apple earns being late

The hardware is what makes me want to play with the Duo, but the software will determine whether I still enjoy it six months later.

Both displays use roughly the same aspect ratio, so applications can move between them without feeling like they have jumped onto an entirely different device. Apple has also moved interface elements such as navigation controls, the Dock and status information toward the sides to preserve vertical space. Even Dynamic Island has been redesigned vertically to fit the new orientation.

When the Duo is open, Split View finally comes to the iPhone. Two applications can run beside each other, and Apple even allows two windows from the same application. I could have two Safari pages open at once, drag a photograph from Photos into Mail, or keep something I am researching on one side while working with Siri AI on the other.

App pairs can be saved and reopened later.

That starts moving the iPhone into territory that previously belonged much more clearly to the iPad. It isn't simply a big phone screen. Apple is acknowledging that once the display reaches this size, people are going to expect to do more than stretch the same application across additional pixels.

I also like what Apple has done with StandBy. Because the Duo can physically hold itself at different angles, StandBy works on either display even when the phone is not charging. I can set it on a desk and have a clock, widgets, calendar or photographs sitting there without needing a stand.

This is where I think Apple has an advantage despite arriving late. Samsung and others have had years to prove that the hardware can work. Apple gets to enter after many of the obvious mechanical mistakes have already been discovered and concentrate on making the shape useful throughout iOS and the rest of its ecosystem.

Whether it has actually succeeded is something I want to judge after living with one, not something I am willing to declare from a product page.

There are real compromises

I am excited about this phone, but I don't think the Duo magically solves everything that has made foldables difficult.

It starts at $1,999 from Apple. The 512GB model is $2,199, 1TB is $2,599, and 2TB reaches $3,199. That is an expensive phone even by modern flagship standards.

It is 11.3 millimeters thick when folded and weighs 254 grams. The camera system loses the dedicated telephoto hardware found on the Pro models. The flexible inner display necessarily uses different materials from the glass surface we are accustomed to on normal phones, and Apple's claims about durability still need years of actual use behind them.

I also want to see the crease with my own eyes. Apple says the nano-texture and display construction minimize its visibility, but nearly every generation of foldable phone has promised a better crease. I'll reserve judgment until I can look across that display under bad lighting and run a finger over the middle myself.

The hidden FaceTime camera will be another thing to test. A clean, uninterrupted display sounds great, but I want to know what image quality Apple had to trade for it.

These are first-generation Apple hardware decisions even if the foldable phone category itself is mature.

That is part of why I find it interesting.

This feels like a new kind of iPhone

Most yearly iPhone upgrades are easy to understand. The processor gets faster. The camera gets better. The display gets brighter. The battery lasts longer. All useful improvements, but the object in your hand behaves fundamentally like the one before it.

The Duo changes the object.

It can be a small one-handed phone, a 7.6-inch canvas, two applications beside each other, its own camera stand, a little FaceTime terminal, a rear-camera selfie monitor, a StandBy display or, later this year, a notebook I can write on with an Apple Pencil.

That is what makes me excited about it.

Not because Apple invented folding screens. It didn't. Other companies deserve credit for spending years building this category and discovering many of its failures the hard way.

I am excited because Apple finally thinks the technology is ready to become an iPhone, and it has redesigned enough of the hardware and software around the fold that this feels like more than an iPhone with a hinge added to it.

I've kept those old Apple devices because each one reminds me of a point when the way I interacted with a computer changed. The original iPhone obviously did that. The iPad did it again. Apple Watch found another place for computing in my life.

I don't know yet whether the Duo will deserve a place beside those devices.

I am definitely going to find out.

Preorders open at 5 a.m. PDT on October 16.

I'll be awake.

Explore iPhone Duo at Apple.

Read Apple's iPhone Duo technical specifications.

Read Apple's iPhone Duo announcement and engineering details.

View iPhone Duo pricing and preorder options at the Apple Store.

Read Apple's Apple Pencil feature comparison.

Read the history of the long-running foldable iPhone rumors.

Read Reuters' history of the foldable-phone category before Apple's entry.