Looking at some apps for both iPhone and iPad, I discovered some of the best of them have placed controls in ways that best match the ergonomics of our hands while holding them, particularly the saddle joint of the human thumb, with its ability to roll in a particular almost-45 degree sweep. In order to access other parts of the screen requires a reach with the index finger or an extension of the thumb to access it.
Thus, it makes sense, when designing mobile touchscreen apps, to pay attention to these activity zones. Put the high-use controls in the Easy zones, and controls that are less used (and certainly those you don’t want accidentally pressed) in the Reach zones.
Tablet Activity Zones
Mobile Phone Activity Zones