Qt Library

KDE uses as its library of GUI tools the Qt library, developed and supported by Trolltech (www.trolltech.com). Qt is considered one of the best GUI libraries available for Unix/Linux systems. Using Qt has the advantage of relying on a commercially developed and supported GUI library. Also, using the Qt libraries drastically reduces the development time for KDE. Trolltech provides the Qt libraries as Open Source software that is freely distributable. Certain restrictions exist, however: Qt-based (KDE) applications must be free and open-sourced, with no modifications made to the Qt libraries. If you develop an application with the Qt libraries and want to sell it, then you have to buy a license from Troll Tech. In other words, the Qt library is free for free and Open Source applications, but not for commercial ones.