a handheld computer is provided that includes a first module combined with one or more modules . the first module includes a housing having one or more coupling surfaces , with each coupling surface including a coupling mechanism . a second module is coupleable to the first module . the second module includes a third coupling surface having a second coupling mechanism for mating with the first coupling mechanism . the third coupling surface is positioned on the second module so as to abut at least partially against the second coupling surface when the first coupling mechanism is mated with the second coupling mechanism .