Key shape and spacing carry discrimination for eyes-free use

What it is

Once the finger is in the key field, it still has to know which key it is on. Tactile discrimination by shape and spacing is carried by silhouette and gap, not by the legend printed on the cap. A long volume rocker next to a round power button, an empty well between them, a raised wall on one side—these write identity in geometry the pad can feel. Failing to find a key at all is a different problem; this one is pressing the neighbor after you have arrived.

Why it happens

Two-point threshold at the fingertip is roughly 2–4 mm. Identical silhouettes with a seam narrower than that read as one continuous surface; the user falls back on remembered order. Stretch one control into a rocker and leave the next as a short pip, and the contour channel splits the classes first. The gap is a second signal: empty space is itself a landmark, and it makes a single press less likely to fire two switches. Local asymmetry—a rib or chamfer on only one side—lets a sweep read “this is the end key.” Printed legends ride vision; they go dark when the eyes are on the road, in gloves, or at night. Shape and spacing keep working there.

Studying it

Use a tactile identification task: vision blocked, a spoken prompt (“volume up / power / shutter”), tactile cues only.

Independent variables: whether neighboring silhouettes match, inter-key spacing, presence of a well or wall, surface-texture contrast, gloves. Dependent measures: confusion matrix, chorded two-key hits, identification time.

Mean accuracy is less informative than which pair collides. Ten identical round buttons on a remote dump errors onto adjacent indices; changing power into a raised bar should drop that row of the matrix, not the whole board. A glance-then-close-eyes protocol measures memory, not tactile discrimination.

Where it stops holding

Industrial remotes molded from one cavity, with even spacing, simply never open the shape channel; vision or counting has to carry identity. Work gloves worsen two-point threshold, so fine texture and shallow chamfers fail first; large silhouettes and wide wells remain. A panel with a single shutter has no discrimination problem. High-contrast icons on a device that is always looked at can mask shape differences—until the same keys are used in the periphery. Lab finger cots are not winter gloves or surgical gloves.

Applying it

• Do not give functionally different neighbors the same silhouette: a rocker beside a pip survives eyes-free use better than two identical discs with different icons.
• Put a well or a palpable rib between pairs that are already being chorded, so one press cannot fire both.
• Do not assign discrimination to ink that will wear off.
• Verify: list confusable pairs, prompt each 15 times eyes-free, plot the confusion matrix. If one pair keeps swapping, change that pair’s contour or gap—do not add a caption to the whole array. Retest in the gloves the product actually meets.

Related

• Same group: C10.01.1 Physical keys can be located and confirmed without looking · C10.01.3 Replacing physical keys with a touchscreen removes eyes-free capability
• Adjacent: C10.09 Eyes-Free Key Layout Zoning · C6.19 Touch Typing and Homing References
• Search: tactile discrimination · key shape · key spacing