The thumb doesn't reach

Every iPhone has a setting that does something odd. Swipe down on the bottom edge and the top half of the screen slides down, in Apple's words, "so it's within easy reach of your thumb."¹ The phone does not ask you to stretch. It brings the top of the screen to you.

The thumb won the phone, but not by reaching. It won by getting the screen to move.

First, that it won. In 2012 and 2013 the designer Steven Hoober and colleagues watched 1,333 people using phones in streets, airports, cafes and buses. Forty-nine percent held the phone in one hand and touched it with that hand's thumb; 36 percent cradled it in both hands and touched with one, nearly three-quarters of them with a thumb; 15 percent typed with both thumbs.² Add it up and about nine in ten were touching with a thumb.

The thumb is a poor pointer. It is short, broad, and hinged near the wrist, so it sweeps an arc rather than covering a rectangle. It won anyway because it is the only digit free while the same hand holds the phone. Availability beat precision.

For years designers drew the consequence as a map: a fan of easy, stretch and hard-to-reach areas laid over the screen, with the far corners marked off-limits, and the rule that important things go where the thumb can sweep. Hoober's 2013 article drew reach charts like that. By 2017 he was telling designers that they still believe in the thumb-sweep charts, that people shift their grip to change what the thumb can reach, and that, given the choice, they "prefer to view and touch the center of the screen."³ Touches are most accurate there; by his figures a target needs about 7 millimeters at the centre and 12 at the corners.³ And people "often scroll content to bring the part they're reading to the middle of the screen."³ Even in 2013 he had seen people change how they held a phone "sometimes every few seconds."²

So the hand does not travel to the content. The content travels to the hand. Controls mostly sit still at the edges: when Apple redesigned Safari for iOS 15, it put the tab bar at the bottom by default "so users can easily swipe between tabs with one hand."⁴ But the things you read and choose from move. Lists scroll to the middle, sheets slide up, menus drop down, and Reachability pulls the top of the screen into range. The desktop's arrangement, a still page and a pointer that travels across it, turned inside out: the pointer barely moves, and the page goes past it.

That inversion favors the gestures a thumb is good at, scrolling, swiping and pulling, over the one it is bad at, aiming. Much of what phones feel like, the endless feed included, may follow from that one constraint: the hand stays put, so the content has to come.