Abstract: An expandable mobile computer system is described. The mobile computer system includes a peripheral expansion interface that allows a large number of different sized expansion peripherals to be created and used with the mobile computer system. A pair of rails forms a first opening of the peripheral expansion interface wherein an expansion peripheral device may be inserted. Large expansion peripherals may extend in a first dimension out of that first opening. The rails also form a second opening along a second dimension orthogonal to the first dimension. Large expansion peripherals may also extend outward from the peripheral expansion interface in the second dimension.