Palm-rejection direction changes with handedness

What it is

Where the palm sits relative to the nib is not isotropic. In right-handed writing the palm edge is usually below-right or below the nib; in left-handed writing, below-left or below. Some people rotate the wrist above the line (overwriters), so the palm runs ahead of the nib. If rejection paints a fixed disk around the nib, or only trusts “the bottom of the screen”, it will miss the palm on the other side or eat legitimate touches from the other hand. Direction has to follow handedness and grip.

Why it happens

The spatial prior is a wedge or offset region relative to the nib: a right-hand wedge points below-right, a left-hand wedge is mirrored. Contacts inside the wedge that are large and slow are palm. Handedness can come from a system setting, from the recent stable palm–nib vector, or from pen azimuth—the shaft usually points away from the palm. A “bottom strip” palm zone fails when the tablet is landscape or when a title is written on the upper half. Overwriters need the wedge flipped forward; underwriters keep it behind. A wrong handedness setting leaves the true palm outside the wedge (canvas drag) and puts the other hand’s taps inside (swallowed). Tracking the relative vector can follow grip, but the first strokes lack samples, so a default handedness is still needed to avoid rejecting in all directions on a cold start.

Studying it

Recruit left-handers, right-handers, and people who call themselves overwriters or underwriters. Have them write and tap tools with the other hand under locked-wrong, locked-correct, and auto-estimated handedness.

Independent variables: declared handedness, true handedness, wedge facing, azimuth as a cue. Dependent measures: missed true palms, false rejects of the opposite hand, strokes until auto-estimate converges, accidental touches when writing on the upper half in landscape.

A mismatch between the system setting and real grip is a required condition; do not assume the user set it correctly.

Where it stops holding

Two people writing at once have no single wedge. Flipping the tablet for someone across the table inverts handedness in screen coordinates. Children switch hands more often, so a fixed setting goes stale. Pure area classification does not depend on the wedge and is robust to a wrong direction, but harsher on fat fingertips—the direction prior is a biased accuracy trade, not free.

Applying it

• Offer left / right (and optionally “palm ahead of the pen”) as a findable setting before writing.
• After a few strokes, revise wedge facing from the palm–nib vector; on a cold start follow system handedness, not omnidirectional suppression.
• Put the other hand’s toolbar outside the wedge; do not stack a right-hander’s frequent buttons in the lower-right writing zone.
• How to check: a left-hander writes a line under default right-hand settings—watch for canvas drag; it should stop after switching handedness. Then tap a button outside the wedge with the opposite hand; it should not be rejected. Repeat a title on the upper half in landscape.

Related

• Same group: C5.04.1 Palm contact is inevitable while writing · C5.04.2 Rejection uses pen prior or contact area · C5.04.4 Over-rejection swallows intentional finger input
• Adjacent: C5.07 Division of Labor Between Pen and Finger · C5.02 Pen Tilt and Azimuth
• Search: handedness · palm wedge · left-handed stylus