Mainframe customers tend to have a very large financial investment in their applications and data. Some applications have been developed and refined over decades. Some applications were written many years ago, while others may have been written "yesterday." The ability of an application to work in the system or its ability to work with other devices or programs is called compatibility.

The need to support applications of varying ages imposes a strict compatibility demand on mainframe hardware and software, which have been upgraded many times since the first System/360™ mainframe computer was shipped in 1964. Applications must continue to work properly. Thus, much of the design work for new hardware and system software revolves around this compatibility requirement.

The overriding need for compatibility is also the primary reason why many aspects of the system work as they do, for example, the syntax restrictions of the job control language (JCL) that is used to control batch jobs. Any new design enhancements made to JCL must preserve compatibility with older jobs so that they can continue to run without modification. The desire and need for continuing compatibility is one of the defining characteristics of mainframe computing.

Absolute compatibility across decades of changes and enhancements is not possible, of course, but the designers of mainframe hardware and software make it a top priority. When an incompatibility is unavoidable, the designers typically warn users at least a year in advance that software changes might be needed.