A copyright holder can set up different licensing terms for different licensees. On the other hand, if the copyright holder has agreed in a license that certain or all rights are granted to a licensee in a certain period and that the rights will not be granted to others during such period of the time, this license is said to be an exclusive license. When the licensor adopts an exclusive license, he or she will not be able to grant the specified rights to other people. The copyright holder himself or herself may also not be able to exercise the rights granted to the licensee. If no such excluding clauses appear in a license, such is said to be a non-exclusive license. Under exclusive license, as the copyright holder grants rights to a single party, the work may not be fully made use of. This may cause disadvantages to the social goods or the development of knowledge and technology. In free software licenses, therefore, licensors will explicitly express that the terms are not exclusive, so that the rights can be granted to different people at the same time. Exclusive license is a closed, excluding relationship made with a specific person or under specific premises and excludes non-licensees from using the work. Non-exclusive licenses, on the other hand, can grant rights to unspecified number of people. Free software programs must therefore be licensed under non-exclusive terms.