Primary actions belong in the thumb zone, not at the top

What it is

Buy, Send, Next, Confirm payment—the primary action of the current task—belongs in the lower third the thumb can sweep comfortably in a one-handed grip, not on the right of the title bar or at the top of the screen. Moving it down is the app answering a taller phone. It is not waiting for the person to turn on OS one-handed mode, and not stretching the whole page. Two-handed tablet grip and the hard-to-reach centre of a large slab are a different geometry. This is where the button that will be pressed over and over sits on a phone held upright.

Why it happens

A primary action is used far more often than a secondary top-bar entry, so a one-handed thumb keeps reaching for it. Every reach outside the arc is a grip shift or a pause, and the sum hurts more than a single unreachable close. The bottom already holds a system gesture bar and a home indicator, so "move down" is not flush with the physical edge. It is the band between the thumb arc and the system gesture: too low leaks into going home, too high is the unreachable middle-top again. Leaving the main button at the top and hoping people switch to two hands or invoke one-handed mode dumps the form-factor cost onto every tap. Tab bars, bottom bars, and a pinned primary are different surfaces of the same policy: the control this step most wants pressed should appear inside the arc.

Studying it

Use zoned tapping and one-handed tasks on device, not button size on a visual mock.

Independent variables: primary on the title-bar right / vertical middle / comfortable thumb band, presence or absence of a bottom bar. Dependent variables: completion without a grip shift, accidental system-gesture hits, task time, rated effort.

A phone flat on a table, tapped with an index finger, is not one-handed reach. Require one hand, standing. Fitts-style tapping can compare throughput across zones; ecological tasks (checkout, send) are what expose collisions with system gestures. Faster taps at the bottom do not mean every control should sink—secondary and destructive actions sometimes sit outside the arc to cut accidents.

Where it stops holding

A destructive action (delete account, empty) in the most comfortable thumb band raises the cost of a miss; keep it off the primary band or require a second confirm. Landscape, or a fold that squares the aspect, moves the comfortable band; do not paste portrait bottom-bar coordinates onto those shapes. Desktop and a mouse have no thumb arc; the downward move is not a cross-form rule. When the IME rises, a bottom bar is pushed or covered; the primary has to travel with the keyboard or "moved down" becomes "gone" during typing.

Applying it

• Put the single primary of the current step in the band the thumb can hit without a grip shift in portrait. Leave the title bar for wayfinding and secondary entries.
• Leave a gap between the primary and the system gesture bar so an upward home swipe does not submit payment.
• Verify a one-handed checkout or send on the longest supported phone. The primary should press without a grip shift and without OS one-handed mode. If the person has to reach the top-right first, the action is still parked in the unreachable zone.

Related

• Within the group: K1.10.1 Taller phones put the top edge out of the thumb's envelope · K1.10.2 OS one-handed mode is a workaround, not a layout solution
• Adjacent: K3.07 One-handed reach degradation on large screens · C2.11 Thumb reach zone
• Search terms: thumb zone · primary action placement · one-handed use