Modules are the basic building blocks of Modula-3 programs. A module is a named collection of declarations, including constants, types, variables, and procedures. A module can make its facilities available to other modules, called clients.
Modules are composed of two pieces: one public, the other private. The public part is called an interface and gives only the types and procedure interfaces that are needed for clients to use the module. The private section, called the implementation or module, fills in the details with procedure definitions and other private declarations that are hidden from clients.