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I have seen that QT framework offers 4 licenses, as shown here: https://www.qt.io/product/features

Quoting:

Commercial: A commercial license keeps your code proprietary where only you can control and monetize your end product’s development, user experience and distribution – securing your intellectual property.

As I understand it, the commercial license gives you complete freedom on what to do with QT. But it is also an overkill for small applications and individual developers, because:

The Qt Small Business license has all the Qt features for mobile, desktop, and embedded development for 499 USD per year.

This leads me to a question: What would happen if I were to buy this license, develop an application, but only pay the license for the time of the development of the application? Would the application need to change to an open-source license after I stop paying the annual fee?

As an alternative to the commercial license, we have the LGPLv3, GPLv2 and GPLv3 licenses.

TL;DR, from what I currently understand, the LGPLv3 license also lets us do what we want, with the following limitations:

  1. Must dynamically link to QT libraries
  2. Cannot use the grayed out features/libraries.

The second limitation I don't understand very well, because design tools are grayed out as well, does this mean I cannot use them, or that I cannot include the actual design tools in my application?

And finally the GPLv2/3 license. From what I understand, I can use most features, but my application needs to be open source and under that same license. Is this correct?

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  • Who is your client and what is your legal system? Why can't you publish your code under LGPLv3+ license? Did you contact your lawyer? If you are a student, consider publishing your code on github.com under LGPLv3+, but ask permission to your teacher before. If you code on Linux, you can use Qt thru build automation tools (like I did in refpersys.org ...)
    – Basile Starynkevitch
    Jun 25, 2020 at 15:40
  • Helllo @BasileStarynkevitch, I don't know anything about laws and don't have a lawyer. I want to make an app for a friend, and was thinking about using QT for the UI. My friend then wants to do whatever he wants with the app (might use if for business in future). If I publish under LGPL then there are certain QT libraries I will not be able to use.
    – MangaD
    Jun 25, 2020 at 16:29
  • Then don't use Qt. Or ask your friend to install a Linux distribution (e.g. Debian ....) on his computer, and use FLTK. Or consider a web approach with Wt. Or ask your lawyer
    – Basile Starynkevitch
    Jun 25, 2020 at 16:31
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    See also this answer. Read what is free software and references in this draft. Free software does not mean "software with no cost". And please edit your question to improve it, don't comment it
    – Basile Starynkevitch
    Jun 25, 2020 at 16:35
  • "Would the application need to change to an open source license after I stop paying the annual fee [for the Qt Commercial license(s)]?" - This part of your question can only be answered by the Qt Company. We can only really answer questions about the open source version of Qt.
    – Brandin
    Jun 26, 2020 at 11:12

2 Answers 2

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Would the application need to change to an open source license after I stop paying the annual fee?

It depends on the the terms; it might mean you get a perpetual licence to use the libraries, but only an annual support and dev tools licence, or it might mean that you have to keep up payments to be entitled to keep selling your product, or it might mean something else. Any way, that is a question about the exact terms of a proprietary licence, and off-topic here.

design tools are grayed out as well, does this mean I cannot use them, or that I cannot include the actual design tools in my application?

Since they are grayed-out for LGPL, but not for commercial or GPL, it seems that if you want to build proprietary software and pay for the licence, you can have them (though on what terms we don't know, and it's off-topic for this site); if you want to build free software, under GPL, then you may also have them, presumably on GPL terms; and if you want to build proprietary software but not pay, by taking it subject to the LGPL option, then you don't get the helpful tools.

And finally the GPLv2/3 license. From what I understand, I can use most features, but my application needs to be open source and under that same license. Is this correct?

That is correct.

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  • "I'd guess that it means you don't even get a copy of them" - But when I installed QT on Arch Linux I did get those apps...
    – MangaD
    Jun 26, 2020 at 16:27
  • Which version - the LGPL one or the GPL one? It looks like I was mistaken, and you do get the tools with the GPL version - presumably, some kind of share-and-share-alike deal. I have amended my answer.
    – MadHatter ♦
    Jun 26, 2020 at 18:25
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You can use LGPL Qt and static linking

The easy way: a QT LGPL app template

https://marketplace.qt.io/products/qt-lgpl-app-template

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