Mobile PCs meet all PC criteria but are designed to be easily moved from place to place. The system is completely self-contained and can be carried as a single unit, which includes a keyboard, a display, mass storage and the main system unit. Its power sources are alternating current (AC) or direct current (DC). Mobile PCs also include tablet devices that are distinguished by a pen-based OS, which uses a pen as a primary input device rather than a keyboard. Mobile PCs include desktop replacement, mainstream, mini-notebook, ultraportable, tablet or other mobile configurations.