A short extract from the book “Designing for Small Screens”.
“Physical interaction Interaction with small-screen devices reveals the conflict of interests between creating the smallest physical size that will give the user unrestricted mobility and flexibility, whilst maintaining dimensions that are defined by the size and the motor functions of the human hand. The balance is not always achieved and some devices are already smaller than the minimum size that can be comfortably operated by an adult hand. There are two fundamental types of physical interaction that can be distinguished: one-handed and two-handed interaction. Some devices can only be operated with two hands, for example, the Game Boy, whilst other devices offer different modes that can be selected alternately. Smart phones, for example allow the user to dial numbers with one hand, but interaction for more complex applications such as using the calendar or accessing the internet is done with two hands.”
Link: Feature: Designing for Small Screens (usabilitynews.com)