It's a pattern that works very well - many large companies have standing policies that they can't use copyleft licensed software so they have to get the paid license while those happy with the copyleft social contract get to enjoy and contribute for free.

Correct, although not every company shies away from copyleft, namely Amazon which will package your AGPL product wholesale and sell it. They are after all not required to open source their infrastructure code.

> package your AGPL product wholesale and sell it.

if anyone has a problem with that, then they have a problem with software being open source.

Or they are discriminatory and just don't want anyone they personally dont like to benefit.

> Or they are discriminatory and just don't want anyone they personally dont like to benefit.

Which is their right as long as they are not discriminating against a protected class, as it's their work, but they should have put that in the license.

Forgive me for a naive question but how do external contributions to the AGPL code find their way back to the commercially licenced code?

Are all contributions to the "original" pre-licenced source? If so is there some extra legalese to ensure that the changes will always end up in the AGPL source too?

I know of many successful projects using dual licencing so it must work, I'm just wondering how on a legal front.

I don't understand all of your questions, but the proprietary part of the "dual license" can be anything the copyright holder wants. Qt is probably the most famous dual-license software. Check https://www.qt.io/pricing for some but not all details.

It looks like this is subscription-based, so I'm guessing you can update for the length of your subscription.

Perhaps you're asking whether you're allowed to use development patches written by external people that you'd maybe find in github discussions etc.? If you need that, you will probably have to negotiate for that option yourself. If that is your question, then that isn't really a naive question at all.

Outbound licenses (say, the GPL or proprietary) and inbound license for contributions are orthogonal. If you intend to dual license outbound, you need to clarify that in your inbound CLA.

The contributor would need to either:

- Agree to a Contributor License Agreement that grants the project owner an irrevocable world-wide commercial license (probably missing a few adjectives but you get the idea)

- Transfer the copyright

- Release to public domain