Clustering key actions on one edge serves a one-handed thumb

What it is

When the other hand is away, the tablet is held in one hand and the thumb can only sweep the edge next to that hand. Clustering the critical actions on that side is for this degraded window: pause, send a reply, dismiss whatever is ringing, all inside the thumb arc, without swapping hands or setting the device down. This is not packing the whole interface onto that side—the default for the whole interface remains two hands. What is clustered are the few actions that must be tapped now, after which the second hand can return. Rotation will move which physical edge that side is; that is the next entry.

Why it happens

In a one-handed hold the supporting hand usually takes the bottom or one long edge, and the thumb arc sits along the inner side of that hand. Anything outside the arc needs a grip change or the other hand. Pause in the far upper corner makes people set the device down, and by the time the absent hand returns the task has already broken. Clustering on one side admits that the only mover available in this window is one thumb, and sends the steps that must finish to where it can sweep. Which side depends on which hand is holding—left hold puts critical actions lower-left, right hold lower-right. Hard-coding the right edge excludes a left-handed hold. If the system can follow the holding hand, the side can move with it; if not, placing the same critical action on both sides is more stable than placing it only on “most people’s right.”

Studying it

Occupy the non-dominant hand (a bag, a rail). Have the dominant hand hold the tablet alone and do pause, reply, dismiss an alarm. Compare critical actions on the near edge, in the center, and in the far upper corner.

Independent variables: target location relative to the holding hand, whether the same action is offered on both left and right. Dependent variables: completion, times the device is set down, grip changes, time.

The near edge should be clearly faster than the far one. Measure left- and right-dominant holds, or a right-hand hot zone will be treated as a universal law. Tasks must be short; long ones are interrupted by weight first, and the study then measures fatigue rather than reach.

Where it stops holding

With both hands present, locking every primary action on one side leaves the other hand idle—that is a degraded layout harming the default. Products with nothing that must finish in a one-handed window (desk-bound creation) need not cluster on a side. Left and right dominance, and portrait versus landscape, change which edge “the side” is; a fixed pixel location goes stale quickly. A folded cover display can use a phone’s bottom one-handed zone; it does not need a new side-cluster.

Applying it

• List actions that may still be needed while the other hand is away (pause, send one message, dismiss, back) and put them in the current holding hand’s thumb arc.
• Do not serve only a right-handed hold: offer both sides, or switch with the holding hand.
• These entries remain tappable in two-handed use, but they must not become the only primary-button location in two-handed use.
• Verify while carrying something, one-handed: pause a video, answer a notification. If the tablet has to be set down, those actions are still on the far side.

Related

• Within the group: K3.07.1 Tablet size puts one-handed reach out of bounds; two hands are the default · K3.07.3 Rotation relocates the one-handed reach zone · K3.07.4 Designing for degraded one-handed use must not tax two-handed work
• Adjacent: C2.11 Thumb Reach Zones · K1.10 One-handed Mode and Large-screen Reach · K3.06 Grip and Occlusion
• Search terms: one-handed thumb · edge clustering · reach envelope