Software Architect's Handbook

Common coupling

Common coupling, also known as global coupling, is a high level of coupling. This type of coupling is highly undesirable. Although sometimes it is unavoidable, modules should be designed to minimize the existence of this type of coupling.

Modules exhibit common coupling when they share the same global data, such as a global variable. It is perfectly acceptable to share configuration data throughout an application. However, as a general rule, if you are going to use other types of global data, it is better to use something that has a fixed value, such as a constant, rather than a variable whose value can vary at runtime.