Apple's slide to unlock sound dates back to the first iPhone, when users dragged a slider across the lock screen to open the device, and the same gentle click remained a familiar part of iOS through version 5 in 2011. When Apple later removed slide to unlock, the sound became a marker of an earlier era of smartphones, preserved in videos, soundboards and edits that reference old iPhones. It still works today because listeners recognise it instantly, even without seeing a phone, and it can set a scene in less than a second.