Public Domain Is Not Open Source

Open Source and Public Domain are frequently confused. Here’s why it’s a mistake to treat the two terms as synonyms.

Plenty of people assume that public domain software must be open source. While it may be free software within your specific context, it is incorrect to treat public domain software as open source or indeed as globally free software. That’s not a legal opinion (I’m not a lawyer so only entitled to layman’s opinions) but rather an observation that an open source user or developer cannot safely include public domain source code in a project.

First let’s look at the two terms and what they mean to individuals.

  • Open Source” describes a subset of free software that is made available under a copyright license approved by the Open Source Initiative as conforming with the Open Source Definition. Having a standards body for licenses — one which ratifies the consensus of an open community of license reviewers — saves individuals from needing to each seek out a legal advisor to tell them whether a given license does in fact give them the rights they need to build or deploy the software they want. By providing easy certainty, open source gives people permission in advance to meet their own needs and innovate with technology.
  • Public Domain” means software (or indeed anything else that could be copyrighted) that is not restricted by copyright. It may be this way because the copyright has expired, or because the person entitled to control the copyright has disclaimed that right. Disclaiming copyright is only possible in some countries, and copyright expiration happens at different times in different jurisdictions (and usually after such a long time as to be irrelevant for software). As a consequence, it’s impossible to make a globally applicable statement that a certain piece of software is in the public domain.


The community goes to great efforts to ensure things called “open source” are under an OSI-approved copyright license even though some regard it as obstructive bureaucracy. Many would prefer to simply say their code is “public domain” but that doesn’t deliver the key benefit open source offers: certainty over the core freedoms of free software. That leaves doubts about there being sufficient permission-in-advance for some collaborators.

It’s easy to get overwhelmed by all the details of the many open source licenses, losing sight of the reason they are important. They’re important because every open source license guarantees the freedom to innovate without seeking permission first. OSI approval means you have the unconditional right to use the software in question for any purpose (sometimes called “freedom zero”). You also have an unconditional right to make new software based on that software for your own use, and a conditional right to share the software — modified or not — with other people. That final case comes with some complexities, but they are considered elsewhere.

Public Domain software may come with the rights delivered by those “four freedoms”, but you can’t be sure. It will depend where the software was written, where you are located, who the author is and where the people you are sharing the software with are located. A deployer or developer will need to at very least ask for advice before proceeding, and most likely will need to secure the services of a legal professional with experience in copyright law in each affected country. Even asking the author is unlikely to be conclusive. That’s why public domain software may be free software but is not certain to be.

A solution would be to create a form of words to be used by the author to dedicate something to the public domain. It could simply disclaim ownership for the jurisdictions where that is possible, and then grant a copyright license that has the same practical effect as a public domain dedication for jurisdictions where ownership of copyright can never be disclaimed. Such a formulation has been published by the Creative Commons. They call it “CC0” and it is widely used and well respected.

There’s an issue though; it has not received OSI approval. Many reviewers believe it should, but its authors withdrew it from the approval process during a complex discussion about patent rights and it remains unapproved. The matter has recently been re-opened; lets hope OSI and CC can work together to bring the certainty on which open source thrives. In the mean time, it’s probably safer to use a license like MIT instead.

This article originally appeared on “Meshed Insights” and was made possible by Patreon patrons.

Image credit:
Only Only” by Open Source Initiative, 2017, CC0 / Public Domain, is a derivative of “Right & Left Turn Only Arrow Sign” by Linnaea Mallette, via is used with permission under CC0 / Public Domain.