Modularity
Modularity is closely tied with encapsulation; think of modularity as a way of mapping encapsulated abstractions into real, physical modules.
The C/C++ convention is to create two files for each class: a header file (.h suffix) for the class interface, and an implementation file (.c, .cp, .cpp, .C suffix) for the code of the class.
Booch gives two goals for defining modules. Make a module cohesive (shared data structures, similar classes) with an interface that allows for minimal inter-module coupling.
Other considerations: team work, security, documentation.
Important to remember that the decisions concerning modularity are more physical issues, whereas the encapsulation of abstractions are logical issues of design.
It is possible to "over modularize". This can increase documentation costs and make it hard to find information.
