A Software Licence is a legal instrument that governs the conditions under which software may be used, copied, modified, and distributed by parties other than the copyright holder. Licences range from permissive open-source agreements such as MIT and Apache 2.0 to copyleft licences such as GPL, and to proprietary end-user licence agreements. The licence type determines the legal obligations of users and downstream distributors with respect to attribution, source disclosure, and commercial use.