## 4 Discussion  
This study found that there were slightly more letter keys that were statistically more often touched by the left thumb (i.e., TUL \(>\) 50.0%) during two-thumb key entry on the QWERTY soft keyboard, regardless of the participant's handedness. For both the left-handed and right-handed participants, the number of letter keys with TUL values exceeding 50.0% was greater than the number of keys with TUR values exceeding 50.0%. In addition, the number of keys with TUL values greater than 85.0% was equal to or more than the number of keys with TUR values greater than 85.0%. To recap, this can be interpreted as that keystrokes on the QWERTY soft keyboard were made by the left thumb relatively more than the right thumb, regardless of handedness. In fact, this propensity was expected because the habits developed from using a physical QWERTY keyboard were likely to have influenced the use of a smartphone QWERTY soft keyboard--this has often been noted in existing studies [19; 20; 21]. In general, a physical QWERTY keyboard tended to be designed to use the left hand relatively more for typing letter keys to compensate for the technical defects of mechanical typewriters [26; 27; 28; 29]. Thus, users have naturally been trained and accustomed to using their left hand relatively more often for typing letter keys on the physical QWERTY keyboard, as illustrated in Figure 5. In considering that QWERTY keyboard layouts of mechanical typewriters were passed down to QWERTY keyboard layouts of computers and furthermore smartphones [21; 30; 31], it was not surprising to see this finding in the present study--the keys that were relatively more often touched by the left thumb in the present study were almost identical to the keys that were specified to be pressed by the left hand in Figure 5, except the key B. Most keys were touched statistically more often using the thumbs that were relatively closer to the keys in the distance, excluding the keys G and V in the middle area. This tendency seemed to weaken the more centrally the keys were on the QWERTY soft keyboard. Namely, the closer a thumb was to a target key, the more likely it was that the thumb could tap the target key. We estimated that this phenomenon was associated with the following human behavioral predispositions. First, this could be affected by the human tendency to choose the most economical trajectory or sequence of movements in terms of energy cost [32; 33; 34]. This was because the participants in the present study may have intended to shorten the travel distance to target keys by choosing a relatively closer one of the left and right thumbs in order to reduce the energy that was required for keystrokes and increase the efficiency of the touch tasks. Second, this finding could be seen as a result of another human tendency to prefer easier ways to complete tasks [35]. This was because the finding can be interpreted as a behavior to lower the index of difficulty to locate the key--considering all the letter keys had the same size in this study, the most effective way to decrease the index of difficulty was to reduce the thumb excursion, according to [PERSON] law [36; 37; 38], i.e., this could be considered a behavior that attempted to make the given task easier by choosing the thumb that was relatively closer to the target key. Interestingly, for the letter keys G and V (i.e., in the middle area) that were equidistant from the preset positions of both the left and right thumbs, overall, their touches were  
Figure 5: The recommended key assignment to the left and right hands on a physical QWERTY keyboard. The key assignment was adapted from [26]. The dotted line indicates the key assignment to the left and right hands. observed to be made relatively more often with the left thumb--these keys shared the TUL values more than 50.0%, except for the key V for the right-handed (the same TUL and TUR values of 50.0%).