Look at the button you press to place or end a call, on a phone that is a featureless slab of glass, and you will find a small curved shape that no longer exists in the room. It is a handset: the earpiece-to-mouthpiece silhouette of a desk telephone from around the 1970s, when a call meant lifting a heavy curved receiver from its cradle and holding it against your head. The object is gone from most desks, but the picture of it stayed exactly where it was.

The reason it stayed is quietly logical. When phones became flat rectangles, they lost their shape entirely, and a shape is precisely what an icon needs. You cannot draw the act of calling from a featureless rectangle, because the rectangle looks the same whether it is ringing, playing music or switched off. The old handset, by contrast, is instantly legible: two round ends and a curved bar, unmistakably a thing you speak into. So the drawing simply refused to modernise, because there was nothing to modernise it into.

This makes the handset a particular kind of survivor, a skeuomorph that outlived its object. It began as a picture of a thing in the world, a faithful little portrait of the receiver on your desk. But somewhere along the way the thing vanished and the picture kept going, and now the icon points not to an object at all but to an action. It no longer means "a telephone receiver." It means "make this call" or "hang up," a verb wearing the costume of a noun that retired decades ago. We tap the ghost of a handset to do something no handset was ever needed for, and everyone understands it perfectly.