Q. Why do foldable phones have a crease, and does the iPhone Duo have one?

Apple says it has made the crease on the iPhone Duo, its first foldable iPhone, harder to see. Open any foldable and you may spot a faint line down the middle of the screen. Why do foldable phones have a crease at all, and what does the hinge have to do with it?

The short answer: folding squeezes the layers on the inside of the bend and stretches the ones on the outside, all along one line, and after enough folds those layers no longer spring all the way back, leaving a faint crease. The hinge decides how tightly the screen bends, so it has a big say in what that crease looks like.

What is a foldable screen made of?

A foldable screen is not a single sheet of glass but a sandwich of thin layers: an OLED (organic light-emitting diode) panel whose pixels make their own light, a clear cover layer on top, and clear adhesive in between. Samsung’s Galaxy foldables also ship with a protective film over everything.

For the cover layer, the first Galaxy Fold, in 2019, used a clear plastic film made of polyimide. Bendable glass followed in February 2020, when Samsung Display said it was the first in the industry to commercialize ultra-thin glass (UTG) for foldables, starting with the Galaxy Z Flip. That glass is 30 micrometers (0.03 mm) thick.

Why do foldable phones get a crease?

Fold a thick notebook in half: the inner pages bunch up, while the outer pages travel farther around the bend and pull tight. A folding screen behaves the same way. Samsung Display says that when its multilayer panel folds, a squeezing (compressive) force builds on the screen side and a stretching (tensile) force on the back.

How tightly it bends matters too. Press a paperback open flat and the spine gets a line; read it half open and it doesn’t. Samsung Display says the smaller the radius of the fold, the more folding stress the layers must survive. Its Galaxy Z Fold2 panel folded with a 1.4 mm radius (1.4R) in 2020, the tightest commercial foldable at the time; the first Galaxy Fold and the Galaxy Z Flip used 1.5R.

Then there is repetition. In a 2025 paper for the Society for Information Display, researchers at glassmaker Corning wrote that the crease grows with repeated folding and with time spent folded. Counterpoint Research, a market research firm, points to how well the adhesives between layers recover as one key to controlling it. Like a letter left folded in its envelope, the screen keeps a little of every fold.

You notice the result as slight unevenness under a finger and as reflections that warp at the fold, which the Corning team compares to a funhouse mirror. Samsung’s support pages say a crease may be visible in the center of the screen.

What does a hinge do on a foldable phone?

A hinge is what lets a door swing: two plates joined by a pin. On a foldable phone it joins the two halves, but it also decides what shape the screen takes when the phone closes.

Enter the waterdrop hinge. According to Korea’s Electronic Times, Samsung’s foldables up to the Galaxy Z Fold4 and Flip4 bent the screen into a tight U, leaving a gap between the closed halves. The Flex Hinge on the 2023 Galaxy Z Fold5 and Flip5 curls it into a teardrop-shaped loop instead, which, as 9to5Google explained, puts less stress on the display and lets the phone close flat.

There is a trade-off: Samsung’s Korean support site says the fold mark on the Fold5 and Flip5 may look slightly wider than on earlier models, a characteristic of the waterdrop design. A hinge does not erase the crease so much as decide its shape.

How durable are foldable screens?

Durability is tested by machines that fold a screen a set number of times while checking that it still works. Samsung Display has had the French certification body Bureau Veritas run its tests:

Announced	Screen tested	Fold test result
Feb 2020	Ultra-thin glass (Galaxy Z Flip)	Passed 200,000 folds
Sep 2020	Galaxy Z Fold2 panel (1.4R)	Passed 200,000 folds
Jul 2025	8-inch (20.3cm) panel for Galaxy Z Fold7	Passed 500,000 folds (13 days at 25°C)
Sep 2026	iPhone Duo	No fold count in Apple’s release

Bureau Veritas said 500,000 folds is 2.5 times the industry standard of 200,000 and equals about 10 years of average use. A fold count tells you how long the screen keeps working, not how visible the crease will be.

Samsung’s support pages also ask Galaxy foldable owners to:

• keep the factory protective film on, since removing or reapplying it can damage the panel
• avoid pressing hard on the fold, or closing the phone on cards, keys or sand
• not press the screen with pens, nails or keys
• not open the phone with a finger between the screens

For the iPhone Duo, follow Apple’s own user guide.

Does the iPhone Duo have a crease?

The 7.6-inch (19.3cm) inner display has a nano-texture finish, a matte surface with microscopic texture that scatters light. It cuts glare and reflections and, Apple says, minimizes crease visibility. Since a crease mostly shows up as warped reflections, fewer reflections make it harder to spot.

The cover layer is a custom polymer with up to 40 percent higher stiffness than other materials used in the industry, plus a scratch-resistant coating. High-strength glass sits above and below the folding panel, and custom adhesives let the layers glide past one another like the pages of a book, relieving bend stress. A titanium plate sits at the bottom, and a hinge built from more than 100 components holds the center of the screen flat when open.

All of this is Apple’s own description, and Apple does not claim the iPhone Duo has no crease. It has not published a fold count either. How visible the crease is, and how it ages, will only become clear from reviews after the 23 October launch.

In short

A foldable’s crease is the lasting trace of folding, which squeezes the inner layers and stretches the outer ones, fold after fold. Waterdrop hinges, better layers and matte finishes can make it smaller or harder to see. Only reviews will show how visible the iPhone Duo’s crease really is.