Apple spent nearly a decade moving the iPhone away from Touch ID. The technology that defined iPhone authentication for years disappeared from Apple’s flagship lineup when the company introduced the iPhone X in 2017. Apple removed the Home button, replaced it with gestures and introduced Face ID as the new way to unlock the phone and authenticate payments. Since then, Face ID has become a standard part of the premium iPhone experience, while Touch ID survived mainly on older designs and the iPhone SE. Apple seemed to have made a permanent choice, but the iPhone Duo, Apple’s first foldable iPhone, has changed that.
The Duo brings Touch ID back in the side button, almost nine years after Apple removed it from its flagship iPhones. This could be seen as a decision made purely because of the limitations of a foldable design. I think it says something more important about Touch ID. Apple did not move away from fingerprint authentication because it stopped being useful. It moved away because Touch ID was tied to the Home button, and the Home button had become incompatible with the all-screen design Apple wanted for the iPhone. Now that Apple is building a device with very different hardware constraints, a fingerprint sensor has become useful again.
The iPhone X was the point where Apple had to make a choice. Touch ID was built into the Home button, which occupied a significant portion of the front of the phone. Removing that button allowed Apple to extend the display further toward the edges and introduce gesture-based navigation, but it also removed the fingerprint sensor. Apple needed another secure biometric system, and Face ID was a natural fit because it did not require a physical button or any direct contact with the device. It allowed Apple to retain biometric authentication while making the display the dominant part of the phone’s front.
Apple then spent years building around Face ID rather than looking for a replacement for it. The TrueDepth camera system became an important part of the iPhone’s hardware, while Face ID was integrated into functions such as unlocking, Apple Pay and app authentication.
The company also continued reducing the amount of space occupied by the front camera and sensors. This made it increasingly unlikely that Apple would bring Touch ID back simply for the sake of offering another authentication method. From Apple’s perspective, Face ID already solved the problem and fit the direction of the iPhone.
What is more interesting is that Apple never followed the path taken by many Android manufacturers and put Touch ID underneath the display. An in-display fingerprint sensor could have allowed Apple to retain fingerprint authentication without bringing back the Home button, but the company never shipped one on an iPhone. There are several possible reasons. Apple had already invested heavily in Face ID, and adding another biometric system would have increased hardware complexity without solving an important problem on conventional iPhones.
In-display fingerprint technology also has its own engineering challenges, including sensor integration with different display technologies and the need for reliable recognition. More importantly, Apple appeared to have a different long-term plan. If it could eventually move the Face ID hardware underneath the display, there would be little reason to spend years perfecting a second biometric system.
The foldable iPhone changes that calculation because the problem is no longer simply how to make the front of a conventional phone look cleaner. The Duo has an outer display and a much larger internal display, along with a hinge and a thin body that has to accommodate batteries, cameras, cooling and other components.
Apple is also putting the inner camera underneath the display, which shows how much importance the company places on keeping the large internal screen free from visible hardware. Finding room for the components required by Face ID becomes more difficult in a design where internal space is already limited and divided between two halves.
Apple could eventually develop a smaller TrueDepth system or move more of Face ID underneath the display, but that would add another engineering challenge to an already complicated product. Touch ID avoids much of that complexity. By putting the fingerprint sensor into the side button, Apple can provide biometric authentication without taking space from either display or placing additional hardware around the front cameras. The sensor also works regardless of whether the Duo is being used folded or unfolded, because its position does not change with the display configuration.
That is why I think the side-mounted Touch ID sensor is a much more logical solution for the Duo than it might initially appear. Apple is not bringing back the Home button or reversing the design philosophy introduced with the iPhone X. It has separated the fingerprint sensor from the physical button that originally made it incompatible with the all-screen iPhone. The sensor can now occupy space that was already needed for a side button, while the displays remain free of additional biometric hardware. In a foldable, that is a meaningful advantage rather than a compromise.
The return also shows something that became easy to forget during the years of Face ID: fingerprint authentication still has some practical advantages. Face ID is extremely convenient when the phone is in front of you and can clearly see your face, but it is not ideal in every situation. A phone lying flat on a desk, for example, can be awkward to unlock with Face ID because the camera needs to see the user. A fingerprint sensor can be used without changing the phone’s position. The same can apply when the device is held at an unusual angle or when part of the user’s face is covered. These are not reasons to declare Touch ID better than Face ID, but they show that the two systems have different strengths.
That could become more important as Apple continues to experiment with new iPhone designs. The company has increasingly focused on thinner devices, and reducing thickness makes every component more difficult to accommodate. Face ID requires several pieces of hardware to work together, while a fingerprint sensor integrated into a side button requires relatively little additional space and does not compete with the display.
If Apple continues making its phones thinner, the side-mounted approach could remain useful beyond the Duo, particularly for products where there is a strong reason to keep the front of the device completely unobstructed.
I would not expect Apple to replace Face ID across its conventional iPhone lineup. Face ID remains well suited to the standard iPhone design, and Apple has invested too much in the technology and the surrounding ecosystem to abandon it without a compelling reason. The more interesting possibility is that Apple could use different biometric systems for different products. Face ID can remain the default on conventional iPhones, while Touch ID becomes an option for foldables or particularly thin devices where its simpler hardware makes more sense.
It is also possible to imagine Apple eventually supporting both technologies on a single iPhone, although I would not expect that simply because it is technically possible. Apple generally avoids adding hardware without a clear benefit, and two biometric systems would add cost and complexity. If Apple eventually manages to move Face ID completely underneath the display, however, Touch ID could provide an additional authentication method without compromising the screen. Whether Apple sees enough value in that combination remains unclear, but the Duo makes the idea far more plausible than it was a few years ago.
The bigger lesson from the Duo is that Apple did not really abandon Touch ID because fingerprint authentication had become obsolete. It abandoned a particular implementation of Touch ID because the Home button no longer fitted the iPhone it wanted to build. Face ID was the right solution for the iPhone X because it allowed Apple to remove that button while retaining secure biometric authentication. The Duo has created a different set of priorities, and those priorities happen to make a fingerprint sensor useful again.
There is a certain irony in that. Apple removed Touch ID because it wanted the display to take over more of the front of the iPhone. Now it is bringing Touch ID back because it wants the displays of its most ambitious iPhone to remain as unobstructed as possible. The technology has returned, but the reason is still the display.
Touch ID therefore does not need to be seen as a rejection of Face ID or a return to an older iPhone philosophy. It is better understood as Apple adapting an existing technology to a new form factor.
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