Different types of free and open source licences

Free and open source software licences can be further classified into copyleft licences and permissive licences. Both provide the same set of freedoms as far as usage, inspection and modification are concerned. But the freedom for redistribution is different for copyleft licences and permissive licences. A copyleft licence explicitly makes sure that a derived or modified work of free/open source software also adopts a licence similar to what accompanied the original software. The GNU GPL is an example of a copyleft licence. Thus, if you adopt GPL for your software, every derivative work will also come under GPL or a similar licence. But remember, copyleft licences are also based on copyright laws. Figure 1 shows the copyleft symbol. The symbol alone has no legal validity and the great Richard Stallman himself criticised its use in many legal documents. Copyleft licences are very strict about retaining the freedoms associated with software,