Proprietary software, also known as closed-source software, is a type of non-free computer software. During the 1990s, it became the dominant model for distributing operating systems and office applications. Its main characteristic is that the source code is not available to the end user, who acquires a usage license. This model contrasts with free software, such as that regulated by the GNU GPL, which allows modification and redistribution. The mass adoption of this type of software marked the professionalization of the technology industry and defined the relationships between developers, companies, and domestic users in the personal computer era.