[karmicthreat]

I found the applicable text: https://www.qt.io/terms-conditions/

*NOTE: If Licensee, or another third party, has, at any time, developed or distributed all (or any portions of) the Application(s) using an open source version of Qt licensed under the terms of the GNU Lesser General Public License, version 2.1 or later (“LGPL”) or the GNU General Public License version 2.0 or later (“GPL”), Licensee may contact The Qt Company via email to address [e-mail] to ask for the necessary permission to combine such development work with the Licensed Software. The Qt Company shall evaluate Licensee´s request, and respond to the request with estimated license costs and other applicable terms and details relating to the permission for the Licensee, depending on the actual situation in question. Copies of the licenses referred to above are located at http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html, http://www.gnu.org/licenses/lgpl-3.0.html, http://www.fsf.org/licensing/licenses/info/GPLv2.html, and http://www.gnu.org/copyleft/gpl-3.0.html.

So really the questionable term isn't an additional term on the LGPL, it is on the commercial license.

[lultimouomo]

That is very interesting. Note that they are not precluding you to switch, but they do reserve the right to sell or not sell to you the commercial license after you've used the LGPL. It sounds like an empty threat to me (if you talk to the sales rep and say "I have an internal prototype and I would like to buy the commercial license to finish and distribute it", will they say no?), but one should definitely consider this before choosing to try the LGPL version for early product development.

I think this is mostly meant to avoid having a team of 50 develop an internal version based on LGPL, and then a team of 1 maintain the commercial version, paying a single developer seat.

Note that you can still do a throw away prototype without problems. Some say you should in any case!

[jschwartzi]

It's not an empty threat so much as it's an opportunity for them to gouge you later when your product is already highly dependent on their product.