The IBM Public License (IPL) is a free software / open-source software license written and sometimes used by IBM. It is approved by the Open Source Initiative and is described as a "free software license" by the Free Software Foundation (FSF). It is incompatible with the GPL because it contains restrictions which are not in the GPL. According to FSF "it requires certain patent licenses be given that the GPL does not require. (We don't think those patent license requirements are inherently a bad idea, but nonetheless they are incompatible with the GNU GPL.)"