iPhone Duo uses side-button Touch ID instead of Face ID due to foldable design

Apple's $1,999 foldable iPhone Duo relies on a side-button Touch ID sensor rather than Face ID. The company says the fingerprint reader works consistently across four folding postures, where a front-facing TrueDepth system would be hard to fit.

• Side-button Touch ID works in four folding postures
• TrueDepth needs IR camera, illuminator and dot projector
• Sensor supports multiple enrolled fingers
• Face ID attention features like keeping the screen on are absent