1.4 Computer Systems are the Same but Different 27 In a modular system, one can usually find several ways to combine modules to implement a desired feature. The designer must at some point choose a specific implementation from among many that are available. Making this choice is called binding. Recalling that the power of modularity comes from the ability to replace an implementation with a better one, the designer usually tries to maintain maximum flexibility by delaying binding until the last possible instant, perhaps even until the first instant that the feature is actually needed. One way to delay binding is just to name a feature rather than implementing it. Using a name allows one to design a module as if a feature of another module exists, even if that feature has not yet been implemented, and it also makes it mechanically easy to later choose a different implementation. By the time the feature is actually invoked, the name must, of course, be bound to a real implementation of the other module. Using a name to delay or allow changing a binding is called indirection, and it is the basis of a design principle: Decouple modules with indirection Indirection supports replaceability.