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Open source software (also called "free software") is software that comes with a license that gives users certain rights. In particular the right to use the software, to modify it, to obtain its source, and to pass it on (under the same terms).
Trolltech supports the open source software concept by providing the Qt Open Source Edition. You can use this edition of Qt to create open source software, i.e. software that is licensed under the GNU GPL or a similarly recognized open source license. Trolltech's support of open source with the Qt Open Source Edition has enabled large successful software projects like KDE, to thrive, with thousands of developers around the world using the Qt Open Source Edition at no cost to themselves. The Qt Open Source Edition is available for Unix/X11, Macintosh, and embedded Linux.
The Open Source Edition can be downloaded from here: http://www.trolltech.com/download/.
It is also possible to develop commercial software using Qt, but to do so you must buy a commercial license for the Professional or Enterprise Edition. If you buy a commercial edition of Qt you can sell your software for any price and with any license you like. Thousands of commercial companies use commercial editions of Qt to develop the products they sell.
If you use the Qt Open Source Edition, there are certain licensing conditions that the GNU GPL imposes on you, to ensure that your users enjoy the freedoms guaranteed by the GPL. Users are entitled to:
These freedoms apply to all the source code for all the modules your software is based on, regardless of whether they have been written by you or by others. The freedoms also apply to any associated interface definition files, and even include the scripts and control files used to control compilation and installation of the executable; otherwise users could not exercise their rights.
This means that you cannot use the Qt Open Source Edition if your software must be built with any modules that impose conditions on you that contradict the conditions of the GNU GPL, including, but not limited to, software patents, commercial license agreements, copyrighted interface definitions or any sort of non-disclosure agreement. In these circumstances you must use a commercial edition of Qt.
See the FAQ for answers to frequently asked questions on Qt Open Source Edition licensing and its implications.
More information on open source software is available online:
Information about Qt Commercial License Agreements is available at http://www.trolltech.com/pricing.html. You can also email email@example.com.
If you are in doubt what edition of Qt is right for your project, please contact firstname.lastname@example.org.