Is Open Source Software a subset of free software?

I was recently asked this and realized I was unsure the exact answer. I understand that free software is not a type of open source software, but I am unsure if the reverse is true or not.

  • What is "a type of software"? – user114 Jun 25 '15 at 19:09
  • @Tichodroma I assume the intent here "Is Open Source Software a subset of free software?" but I agree the question could be a bit clearer. – apsillers Jun 25 '15 at 20:20
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    Could you specify what "target audience" for the terminology you are asking about? While the open source community puts a lot of weight on the combination of the free as in free beer and free as in freedom meanings of "free", I suspect the understanding of the general public differs significantly in that "free software" is simply any software that can be obtained free of charge, open source and closed source alike. Presumeably, that is how the WP's disambiguation page on the topic came into being. – O. R. Mapper Sep 23 '15 at 22:25

From Open Source Initiatives FAQ:

"Free software" and "open source software" are two terms for the same thing: software released under licenses that guarantee a certain, specific set of freedoms.

Open Source arose from people who supported the FSF, but later branched out over the decision of philosophy and marketing. There are differences, of course, such as the FSF has a 4-point definition for software freedom, while the OSI has 10. The FSF is sort of like a parent to the OSI.

Generally, we refer to software that is with Free Software and Open Source to be FOSS (Free and Open Source Software), or FLOSS (Free/Libre/Open Source Software)

Pandya raised this in chat: Looks like something where the FSF doesn't agree with Open Source.

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    not to take away from this answer, but there are members of the free software camp would answer completely opposite; as in, open source != free software – albert Jun 25 '15 at 2:20
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    @albert It's true. It's like the enemies that are of the same kind... almost. – Zizouz212 Jun 25 '15 at 2:21
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    @alvert: I doubt any of said camp would say that any open-source licensed software isn't free software, they only claim the term open-source is badly chosen and detracts from the idea of free software. – Mnementh Jun 25 '15 at 9:16
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    @Mnementh just put it awesomely. :D – Zizouz212 Jun 25 '15 at 10:54
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    @vonbrand - nonsense. CDDL is not compatible with the GPL, but the FSF still regard it as a free software license (because it is). GPL compatibility is NOT required for a license to meet FSF's Free Software definition. – cas Apr 15 '16 at 0:07

tl;dr: No.

It depends on the definitions of the terms "Open Source Software" and "Free Software".

A common definition (and in my opinion it’s the one that should be used, otherwise the scope of the terms will be subjective):

In most cases, a Free Software license is also an Open Source Software license, and vice versa. But there are exceptions.

For a definite and up-to-date answer, you should use the original sources and compare if both organizations, the FSF and the OSI, have approved each license:

In the Wikipedia article "Comparison of free and open-source software licenses", there is an "Approvals" table where you can see this at a glance (but this information might not be accurate).

Example 1: Free Software, but not Open Source Software

Netscape’s early versions of Mozilla were released under the Netscape Public License version 1.0 (see its Wikipedia article).

This license is approved by the FSF, but it is not approved by the OSI.

↳ So these early versions of Mozilla are Free Software, but not Open Source Software.

Example 2: Open Source Software, but not Free Software

Active Agenda is licensed under the Reciprocal Public License.

This license is approved by the OSI (currently in version 1.5), but it is not approved by the FSF (it’s listed as unfree, linking to version 1.3).

↳ So Active Agenda is Open Source Software, but not Free Software.


The main difference is philosophical, and so subtle that it is easy to miss because it is not in any licence or list. It is that open source tends emphasize quality, reviewability, correctness and community whereas free software focuses on personal liberty, speech, and opportunity. Both camps endorse the others goals as good, just not quite as important to software and share methodologies and the belief that the ability to see, modify, and distribute source code is critical to their goals. Many projects have people from both camps as well as people who just contribute code and leave the licences to someone who cares.


I think technically speaking it's more likely that "free software" is a subset of "open-source software" because the freedoms of modification and distribution of such, as defined by FSF's four essential freedoms, require the access to the source code.

Nevertheless, as two institutions in the reality, FSF and OSI may have different criteria for software licenses which result in counterexamples to the above technical relationship.

  • Some decade long, motif was close to be open source. Universities could get it, they only required to sign strong NDAs. I think this was nearly open source, but not free (as in freedom). – peterh - Reinstate Monica Dec 25 '20 at 11:43

Given that the OpenSource Definition lists 10 points and the "free" software definition lists only four is a hint that free software is a smaller subset of OpenSource and may violate the OSS rules.

Given that the OSI in former times listed the GPL as a non-OpenSource compliant license before the FSF assured that the ambiguous parts of the GPL have to interpreted in a way that is compliant to the OSS definition creates the impression that the free software definition is a weaker definition.


Different organisations attempt to determine the terms "open source" and "free software. The defintions are often similar in basic spirit but differ in the details and interpretations and are not a strict superset or subset of each other.

And none of these orgnaisation have a monopoly over the english language. So individuals may use the terms in ways that differ from what any of the well known organsiations say. Particulally those outside the "free software community" may use "free software" to reffer to software that is free of charge but may be heavilly incumbered.

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