The Powerbook Duo was the first "dockable computer." This meant that it had a small set of peripheral ports on the computer and relied on the a dock, or device that it would attach to, for a more complete set.

The docks ranged from a simple attachment that would replicate scsi ports and serial ports, to more elaborate devices that would have slots for expansion cards, internal hard drives and even math co processors.

It's main point of downfall is a lack of upgrade options. Ram could be added and it had one internal slot that was specially made for a modem. No other upgrades could be done.