Scissors

A scissor is a bigram where adjacent fingers move in opposing flexion-extension directions, with the shorter finger extending while the longer finger flexes. See scissor terminology for more details.

Scissor Bigram

A scissor bigram is a bigram that creates a scissor motion between adjacent fingers. Scissors are classified by distance:

• Half-scissor bigram (HSB): 1u distance between the two keys
• Full-scissor bigram (FSB): 2u distance between the two keys

$\text{HSB}\% = \frac{\text{\# of half-scissor bigrams}}{\text{\# of bigrams}}$

$\text{FSB}\% = \frac{\text{\# of full-scissor bigrams}}{\text{\# of bigrams}}$

$\text{Total Scissor}\% = \text{HSB}\% + \text{FSB}\%$

Scissors are considered uncomfortable due to the opposing motion of adjacent fingers, which can strain the tendons and reduce typing fluidity. Modern layouts often optimize to minimize scissors, though there is significant individual variation in scissor tolerance.

Note

Extended scissors (3u+) are theoretically possible but rare in standard typing due to corpus sanitization excluding numbers and most symbols.

Finger Pair Classification

Different finger pairs have varying degrees of independence, affecting scissor severity:

• Index-middle scissors: Generally most tolerable due to higher finger independence
• Middle-ring scissors: Moderate severity
• Ring-pinky scissors: Generally least tolerable due to lower finger independence

Scissor Skipgram

A scissor skipgram is a skipgram that creates a scissor motion between adjacent fingers. Like bigrams, these are classified by distance:

• Half-scissor skipgram (HSS): 1u distance between the two keys
• Full-scissor skipgram (FSS): 2u distance between the two keys

$\text{HSS}_1\% = \frac{\text{\# of 1st order half-scissor skipgrams}}{\text{\# of trigrams}}$

$\text{FSS}_1\% = \frac{\text{\# of 1st order full-scissor skipgrams}}{\text{\# of trigrams}}$

Scissor skipgrams capture uncomfortable patterns where the scissor motion happens with an intervening keypress, such as “c_r” in “car”. While less severe than direct scissors, they can still impact typing comfort and rhythm.

Sources

“Scissors,” Keyboard Layouts Doc (3rd edition)