Frequent and primary actions belong in the lower-screen arc

What it is

Send in a chat, like on a short video, add-to-cart on a product card: hundreds of taps a day. Parked as a text button in the top bar, each tap asks for a grip change or the other hand. Primary action in the thumb arc means the control that is both most frequent and most decisive for the task sits in the lower-screen arc that is already reachable and glanceable, so one hand can finish it.

“Primary” is the task outcome, not visual weight. A large brand mark is not the primary action. A visually quiet Send often is.

Why it happens

In a continuous one-handed task the thumb has a default rest, usually lower on the grip side. Short arcs from that rest can be practised toward automatic; a pull from that rest into the upper third is a stretch or a slide of the device, every time. Stretch lowers aim; sliding the device unsettles the whole grip. Both raise the unit cost of a frequent action. A hundred Sends in a day are a hundred stretches, not one.

A large button in a tab bar or along a card’s lower edge is a layout purchase of that short arc. A small top-bar label is a layout refusal of it.

Where it stops holding

On desktop, and on a tablet with a keyboard, the pointer does not start from a thumb rest; primary actions can follow the reading origin or the window structure, and “must sit in the lower half” does not hold. In landscape the lower half is short, and the keyboard or home indicator eats more of the arc; the primary action has to sit in the strip above the keyboard, not flush with the bottom. A device in a dash or kitchen mount, tapped with an index finger, does not have a thumb-arc hot zone. Reachability features that make the whole screen tappable must not be the reason a product’s default placement lives in the stretch.

Applying it

• Name the last tap of each core task (send, pay, add, publish) and check that it sits in the lower-screen arc. If it does not, give it a bottom path no weaker than the current entry.
• A tab bar holds frequent navigation plus at most one primary. Five “all important” entries in the same arc erase the primary.
• In a card feed, keep add/buy on the card’s lower edge; do not reveal it only as a top-bar control after opening the detail page.
• How to check: one hand, walking or slouched, ten repetitions of that primary action. A hand switch, a device slide, or the other hand coming in to steady it means the slot is not in the arc.

Related

• Same group: F1.04.1 The thumb zone is reachable arc times visual attention, not the reachable area itself · F1.04.3 The top diagonal holds only rare, reversible entries
• Nearby: E5.02 Tab bars · F1.03 Proximity and saccades
• Search terms: primary action in thumb arc · lower-screen CTA · one-handed use