Four flavours of Qt now exist, supporting various platforms, but with varying licenses:
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Controversy erupted around 1997 when it became clear that KDE was going to become one of the leading desktop environments for GNU/Linux. Based on the Qt Toolkit, which was then under the Q Public License (QPL), many people in the open source and free software movements were worried that an essential piece of one of their major operating systems would be under commercial control.
This gave rise to two efforts: the Harmony toolkit which sought to duplicate the Qt Toolkit under a free software license and the GNOME desktop that was meant to supplant KDE entirely. The GNOME Desktop used the Gtk+ toolkit which was written for the GIMP, and mainly uses the C programming language. Gtk+ endeavors to have the same cross-platform capabilities as Qt.
Compromises were sought between KDE and Trolltech wherein Qt would not be able to fall under a more restrictive license than the QPL, even if Trolltech was bought out or went bankrupt. This did not satisfy all the critics, and finally, on September 4, 2000, Trolltech announced that the upcoming Qt 2.2 release will be licensed under the GPL. Qt 2.2 was released on September 6, 2000, KDE 2.0 followed on October 23, 2000.
Subsequent releases of Qt/X11 have also been released under the GPL.