History of KDE

Qt, the GUI toolkit used to create KDE applications was previously not under a free software license. The main consequences of this were that proprietary programs created with KDE required a license from Trolltech, the creators of Qt, and any upgrades to Qt had to go through Trolltech. As KDE is an open-source project, the choice to use Qt created controversy among open-source purists. Qt has been re-licensed under the GPL and these arguments have become obsolete, but remain a major part of KDE's history.

Why did KDE choose Qt?

Qt is the best GUI toolkit available for the UNIX platform. The success of a desktop environment depends critically on the availability of applications. Using Qt would push forward application development, and thus KDE.
Qt is available free of charge to anyone, including its source, for free software development and distribution.
We actively encourage commercial software companies to develop applications for KDE. Those companies need a mature, high quality toolkit and technical support. None of the GPL'ed toolkits would have been able to meet these stringent requirements.
Creating an LGPL'ed version of the Qt toolkit that matched the quality of Trolltech's implementation would take time and was deemed lower priority than creating a usable, free, open desktop first.

The KDE Free Qt Foundation

To tackle this problem, the KDE Free Qt Coundation was founded by the KDE project and Trolltech. This foundation ensured the continued availability of Qt as a free software development framework. Namely, should Trolltech no longer make Qt available for free software, the currently released version will be re-released under the BSD style license - a license similar in nature but less restrictive than the GPL or LGPL.

This decision to re-release would be moderated by a team of 2 members representing the KDE foundation (unaffiliated with Trolltech) and 2 members from Trolltech to vote. In case of a tie, the KDE vote will gain higher weighting.

The Harmony Project

Those who prioritised the release of an GPL licensed formed the Harmony project - a project to clone Qt under a GPL license. Rapid progress was made during its time and thus gave purists no excuse to reject KDE as a respectable an compliant desktop environment. However this project was depreciated as Trolltech made the decision to re-license Qt under the GPL.