## 4 Discussion
Above all, we predicted that this was because the keystroke habits developed from using a physical QWERTY keyboard affected the use of a smartphone QWERTY soft keyboard, as discussed earlier. The rationale was that the keys G and V were located in the region that typically encourages typing with the left hand when typing on a physical QWERTY keyboard, as shown in Figure 5. I.e., this means that such a habitual experience may have led the participants to involuntarily touch the keys G and V with their left thumb in the current study. Meanwhile, simultaneously, we estimated that the reason why all the keys in the middle area were not unilaterally tapped by the left thumb would be related to the layout difference between a physical QWERTY keyboard and a smartphone QWERTY soft keyboard. To be specific, the keys G and V of physical QWERTY keyboards are located slightly left from the center (Figure 5), while the keys G and V of typical smartphone QWERTY soft keyboards are positioned at the very center column (Figure 1). That is, as compared with typing on physical QWERTY keyboards, users could likely feel less distance penalty for using the right thumb to touch the keys G and V on a smartphone QWERTY soft keyboard because the keys were moved closer to the right thumb than before. Thus, we determined that this would be a plausible cause of the noticeable increase in the use of the right thumb for tapping the keys G and V in this study. TUL or TUR values between 50.0% and 85.0% were also observed on the keys B and Y for the left-handed participants and on the keys B and T for the right-handed participants--these keys were touched statistically more often by the thumbs that were relatively closer to the keys in the distance. As shown in their TU values (i.e., 50.0% \(\leq\) TUL or TUR \(<\) 85.0%), we should note that the touches on these keys were not made unilaterally with the relatively closer thumbs. That is, this indicated that a noticeable percentage of touches occurred with the thumb that was relatively more distant from the keys as well. We reviewed the possible reasons for this phenomenon. First, in the case of key B, we predicted that such an increase in its TUL values was associated with the trained experience of using a physical QWERTY keyboard, as described above. This was because the key B was originally included in the region on a physical QWERTY keyboard, typically recommended to press it with the fingers of the left hand, as illustrated in Figure 5. Namely, this means that these trained experiences from using a physical QWERTY keyboard may have caused the participants (regardless of handedness) to occasionally press the key B on the QWERTY soft keyboard with their left thumb even though the key B was located slightly off-center to the right, in the present study. Next, for the key Y for the left-handed and the key T for the right-handed, it was unclear why there was such an increase in the number of touches made by the relatively distant thumbs from the keys. Simply, given the scope of this study, it was inevitable to assume that this was vaguely due to the participants' handedness. We just estimated that the participants may have instinctively touched those keys often with their dominant thumb because the keys T and Y were located in the very center column of the QWERTY soft keyboard, which was likely fuzzy in distance perception. However, this is a conjecture, and thus, further study is needed to clarify this. Although significant differences between the TU values of the longer and shorter thumbs in both hands were observed in some cases, as shown in Table 2, there was insufficient evidence to conclude that this finding would have a significant impact on the choice of the thumb to be used for keystroke for the following reasons. First, no consistent pattern was found among those significant differences.