The top diagonal holds only rare, reversible entries

What it is

A tall phone in a right hand: the upper-left corner is the end of the stretch. Reaching More takes the thumb to the joint’s limit, and aim goes loose. Settings, an overflow, a share that is unused for weeks can live there — a miss is reversible, and the day does not ask for many of them. Confirm transfer or Delete album in the same corner is the least stable posture executing the least reversible action.

The constraint is a conjunction: rare, and reversible. One clause is not enough. An occasional Empty is still dangerous. A More opened ten times a day is reversible and still turns the stretch into a daily tax.

Why it happens

The diagonal is the far end of the reachable arc. Arrival pushes the carpometacarpal joint toward its limit, the grip loosens a little, and the landing variance of the contact grows. Larger variance means the neighbour is easier to hit. If the neighbour is Close or Not now, a miss backs out. If the neighbour is Confirm payment, a miss is a transaction.

The stretch also moves gaze. Seeing the upper-left often means sliding the device down or lifting the eyes, tearing attention off the content. That extra attentional cost is acceptable for an entry used once in a while, and unacceptable for a confirmation that must be checked against an amount — the digits and the button are split across an awkward posture.

Where it stops holding

Two-handed video watching, tapping the upper-left with the other hand, is not a stretch zone. For left-handers the dangerous corner is upper-right; pinning Close there as a universal rule hands them the dangerous corner. A tablet held in landscape reshuffles the four corners; the portrait-phone mapping “upper-left = far” does not travel. A system-mandated close (the × on a modal) cannot be moved, so the × itself has to be reversible — it dismisses a layer, not data.

Applying it

• Keep the top corner for overflow, settings, filter entries: rare and backing-out. Move confirm, pay, delete off the diagonal, into the lower half or into a flow that names its object.
• Do not neighbour an irreversible action onto a diagonal entry. If the top bar holds both … and Send, Send should become a primary and move down, or at least take a full word and more gap.
• Even when a transfer or a delete is entered from the top bar, the tap that actually commits should land where the user is already aimed at the content, not on the diagonal itself.
• How to check: one-handed, non-dominant hand, ten taps on the diagonal entry, and look at the landing spray. If the spray intersects any irreversible hit target, the corner is over-assigned.

Related

• Same group: F1.04.1 The thumb zone is reachable arc times visual attention, not the reachable area itself · F1.04.2 Frequent and primary actions belong in the lower-screen arc
• Nearby: C2.11 Thumb reach · F1.07 Spatial isolation of destructive actions
• Search terms: stretch zone · hard-to-reach corner · one-handed reach