Thumb zone: where one-handed reach decides layout

The definition. Holding a phone in one hand, the thumb can comfortably reach only part of the screen. That reachable area divides roughly into three zones: the easy zone, where the thumb's natural curl lands, generally the middle and lower part of the screen on the holding side; the stretch zone, reachable only by extending the thumb or shifting the hand; and the hard zone, the top and the far corner, reachable only by switching hands, using both hands, or sliding the whole hand up.

Why the zones exist. The root is grip posture and thumb length: a one-handed thumb sweeps an arc from the bottom corner of the holding side, and what the arc covers is the easy zone. Screens keep growing, so the hard zone keeps growing with them. The zones are relative, not fixed pixels: hand size differs, and left and right hands mirror each other, so the hardest corner flips, top-left for right-handed users, top-right for left-handed users.

How to use it. One rule: place by frequency and danger. Frequent actions go in the easy zone, occasional actions in the stretch zone, dangerous actions (delete, sign out) deliberately in the hard zone, where distance acts as natural protection against mis-taps. The top holds display content: titles, status, progress, not primary actions. Early iOS putting the back button at the top-left was the classic violation; gesture navigation later routed around it.

Scope, stated first. The thumb zone governs the phone one-handed form only. Tablets are held with both hands, desktops use a mouse; this rule does not apply to them. On Lucerna's phone reading screen, the highest-frequency actions, looking up a word, turning a page, saving to the vocabulary book, belong in the bottom easy zone; low-frequency items at the top of the shelf, search, settings, account, belong at the top. A bottom navigation bar crammed with five buttons drives thumb mis-taps up at once; fewer is better.

Relation to Fitts' law. The thumb zone sets distance, Fitts' law sets speed: the larger the target and the shorter the distance, the faster the hit. Together they are complete: a large button in the easy zone is fastest, while even a large button in the hard zone still costs the hand shift first.