Finger span of chords and one-handed completability
Aliases: chord span · one-handed shortcut · pinky stretch
What it is
A modifier-plus-letter chord looks like two keys. Completing it depends on span: whether the fingers can reach both at once. Control in the lower left plus a letter on the right, or Shift plus a key far from the pinky, often exceeds one hand’s comfortable stretch and needs the other hand. One-handed completability is therefore not “were only two keys involved?” It is whether this hand, on this keyboard geometry, can hold the modifier layer and the target together.
Why it happens
Modifiers live in corners; target letters are sprinkled by word frequency across the main block. While the pinky holds Control or Shift, the reach envelope of the other fingers shrinks: the index still reaches C, V, and X, but not P, L, or the number row. Span can be estimated as key-pitch times key count, and it also depends on whether the wrist leaves home row and whether a right-hand modifier exists. Laptops shrink and drop the left Control, worsening the envelope; a full-size board’s right Control / Alt lets the right hand close some chords locally. When a mouse occupies one hand, span stops being comfort and becomes possibility: the chord must close on the remaining hand. A quasi-mode requires the modifier to stay down; people whose span is short release it to reach the letter, and the chord breaks.
Studying it
On a real keyboard, label each published chord as left-hand-closable, right-hand-closable, or necessarily bimanual, then have people execute them while the other hand holds a mouse. Independent variables include keyboard size, presence of both-side modifiers, and hand length; dependent measures include one-handed success, wrist excursion from home row, and switches to the menu. Testing shortcuts on a full-size desktop board with both hands free writes many “needs two hands” chords down as one-handed.
Where it stops holding
Split, ortholinear, and thumb-cluster modifier layouts rewrite the span map; desktop QWERTY figures do not transfer. Touchscreen chords are almost never two held keys; one-handed completability then follows the thumb’s heat map. People who park the left pinky on Control as a habit can mask span pain; they do not represent smaller hands or a busy other hand.
Applying it
- For commands used while a mouse is still held, choose chords the left hand can close (modifier plus a left or centre letter). Reserve wide-span chords for when both hands are on the keyboard.
- Mark “needs two hands” in the shortcut editor, and walk chords on a laptop with one hand plus a gripped mouse.
- Measure one-handed success of published chords; if failures cluster on right-side letters, rebind rather than asking users to put the pointer down.