Clockwise from top: The logo of the GNU Project, the Linux kernel mascot Tux, and the BSD Daemon Free software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions only to ensure that further recipients can also do these things. To make these acts possible, the human readable form of the program (called the source code) must be made available and the software must be accompanied by a software licence saying that the copyright holder permits these acts (a free software licence). Software can also be free software by putting the source code in the public domain.[1] Alternative terms for free software have been coined in an attempt to make the use of "free" less ambiguous. The most common is "open-source software", which has since evolved to refer to a subtly different sense of freedom. Free software is also known as "software libre", "free, libre and open-source software" ("FLOSS"), and "free/open-source software" ("FOSS"). Free software and "freeware" should not be confused. Freeware is software made available free of charge. Everyone is free to sell copies of free software, to use it commercially, and to charge for distribution and modifications. Because anyone who has a copy may distribute the software at no cost, the software generally is available at no cost. Free software business models are usually based on adding value such as support, training, customisation, integration, or certification. At the same time, some business models which work with non-free software are not compatible with free software, such as those that depend on a user having no choice but to pay for a license in order to lawfully use a software product. The free software movement was launched in 1983 to make these freedoms available to every computer user.[2] Software that does not provide these freedoms is referred to as proprietary software or non-free software.