The licensing spectrum
Key takeaways Licensing is a spectrum, not two camps — it runs from fully proprietary to public domain, with meaningful bands in between. Source-available ≠ open source — visible source can still come with heavy restrictions. Open source splits into permissive and copyleft — both grant the core freedoms, but copyleft adds share-alike conditions. Public domain is the free extreme — no copyright, no conditions at all.
People tend to talk about software as either “open” or “closed,” but that misses most of the picture. Licensing is better understood as a spectrum, running from the most restrictive (locked-down proprietary) to the most free (public domain), with several distinct bands along the way. This lesson lays out that spectrum end to end and gives you one big comparison table to anchor it. By the end you’ll have a mental map that the rest of the path fills in band by band.
The spectrum, end to end
From most restrictive to least, the five main bands are:
- Proprietary / closed-source — source hidden, rights tightly restricted.
- Source-available — source visible, but use is restricted.
- Permissive open source — full freedoms, minimal conditions.
- Copyleft open source — full freedoms, but with share-alike conditions.
- Public domain — no copyright, no conditions at all.
Notice the spectrum has two axes hiding inside it: can you see the source? and what may you do with it? They don’t move in lockstep — source-available shows you the code without granting the freedoms, which is exactly why “I can see it” tells you almost nothing about “I can use it.” Keep the four actions from lesson one in mind — use, copy, modify, distribute — because each band answers them differently.
Band 1: Proprietary / closed-source
Proprietary software is the default-restrictive end. The source code is kept secret (protected as a trade secret), and the license grants you a narrow right — usually just to use the binary, often on specific terms, with no right to copy beyond your license, modify, or redistribute. Most commercial desktop and enterprise software lives here. This is the world of EULAs, seat licenses, and subscriptions, which we cover in Proprietary & closed-source licensing.
Band 2: Source-available
Source-available software lets you read the source — and sometimes modify it for your own use — but the license still imposes real restrictions that disqualify it from being open source. Common limits include “non-commercial use only,” “you may not compete with us,” or “you may not offer this as a hosted service.”
This band has grown fast as companies look for a middle ground: transparency and the goodwill of visible code, without giving competitors the right to take it. But it is not open source, and calling it that is misleading. We give this band its own lesson, Source-available & “fair source”. The key takeaway now: seeing the source is not the same as being free to use it.
Band 3: Permissive open source
Now we cross into genuine open source, which begins with permissive licenses. These grant the full open-source freedoms — use, copy, modify, distribute, for any purpose including commercial — with only minimal conditions, typically just “keep the copyright and license notice.” The MIT, BSD, and Apache 2.0 licenses are the classic examples.
The defining trait: you can take permissively licensed code, modify it, and ship it inside a closed-source product without being required to release your own source. The permission flows out and asks very little back. GopherTrunk uses one of these — Apache 2.0 — which is why others can build on it freely. We compare these in MIT & BSD and Permissive vs copyleft.
Band 4: Copyleft open source
Copyleft licenses grant the same core freedoms as permissive ones — but attach a powerful condition: if you distribute the software (or a modified version), you must release your source under the same license. The freedoms are designed to propagate: everyone downstream gets the same rights you did.
This is sometimes called “viral,” though that word overstates it — copyleft obligations trigger on distribution, not merely on use, and the scope depends on the specific license. Strong copyleft (the GPL) reaches broadly; weak copyleft (LGPL, MPL, EPL) is more contained, typically applying only to the licensed component itself; and the AGPL extends the obligation even to software offered over a network. Copyleft is still firmly open source — you can sell it — but it constrains how you can combine and ship it.
Band 5: Public domain
At the far end, public domain means no copyright applies at all. There’s no owner exercising exclusive rights, so anyone can do anything — use, copy, modify, sell, relicense — with no conditions and no need to credit anyone. It’s even freer than a permissive license, which still asks you to preserve a notice.
The catch is that getting to public domain deliberately is harder than it sounds: in many countries you can’t simply abandon your copyright, and there’s no clean legal mechanism to do so. That’s why dedicated tools exist — the Unlicense and Creative Commons CC0 try to achieve a public-domain-like result, with a permissive license as a fallback for jurisdictions that don’t allow true dedication. We cover them in Public domain, Unlicense & CC0.
The whole spectrum at a glance
Here is the spectrum across the questions that matter most:
| See source? | Modify? | Redistribute? | Sell it? | Main obligation | |
|---|---|---|---|---|---|
| Proprietary | No | No | No | No (only the vendor sells) | Pay; obey EULA; don’t reverse-engineer |
| Source-available | Yes | Often, for own use | Restricted | Usually no | Obey the (non-open) restrictions |
| Permissive OSS | Yes | Yes | Yes | Yes | Keep the copyright/license notice |
| Copyleft OSS | Yes | Yes | Yes | Yes | Share your source under the same license when you distribute |
| Public domain | Yes | Yes | Yes | Yes | None |
Read down any column and the spectrum comes into focus: rights generally expand as you move from proprietary toward public domain, while obligations shift from “pay and obey” to “just keep a notice” to “nothing.” The interesting wrinkle is copyleft, which grants broad freedoms yet adds an obligation back — its share-alike condition is the price of keeping those freedoms open for everyone downstream.
Using the map
This spectrum is the scaffolding for the rest of the path. Most of the upcoming modules zoom into one band:
- The open-source module dives into permissive and copyleft licenses one by one.
- The proprietary & commercial module covers the closed and source-available bands and how to sell software.
- The using open source in products module is all about combining bands safely — what happens when permissive, copyleft, and proprietary code meet in one product.
Whenever you meet a new license, your first move is to place it on this spectrum. That single act — “which band is this?” — tells you most of what you need to know before you read a word of the fine print.
Quick check: a project's license lets you view and read the source but forbids commercial use. Where does it sit on the spectrum?
Recap
- Licensing is a spectrum — from fully proprietary to public domain, with distinct bands rather than a simple open/closed split.
- Two hidden axes — “can you see the source?” and “what may you do with it?” move independently.
- Source-available is not open source — visible code can still carry heavy use restrictions.
- Open source = permissive + copyleft — both grant the core freedoms; permissive asks little back, copyleft adds share-alike conditions on distribution.
- Public domain is the free extreme — no copyright, no conditions, though reaching it deliberately needs tools like CC0.
- Place every license on the map first — identifying the band is the fastest way to understand a new license.
Next up: the same concepts shift across borders — moral rights, registration norms, and which clauses actually hold up. See Licensing across jurisdictions.
Frequently asked questions
Is 'source-available' the same as open source?
No, and conflating them is a common mistake. Source-available means you can see the source, but the license still restricts what you can do — you may not be allowed to use it commercially, compete, or redistribute freely. Open source requires the freedoms to use, modify, and redistribute for any purpose. Visible source is necessary for open source but nowhere near sufficient.
If something is open source, can I sell it?
Yes. A core requirement of open source is that the license can’t forbid selling the software or charging for it — there’s no ‘non-commercial only’ open-source license. What copyleft licenses can require is that if you distribute it, you also share your source under the same terms. Selling is fine; locking it up may not be.
Where does public domain sit on the spectrum?
At the far ‘most free’ end. Public domain means no copyright applies at all — anyone can do anything with the work, with no conditions and no need for permission. It’s even more permissive than a permissive license, which still imposes small conditions like keeping a notice. Reaching public domain deliberately is trickier than it sounds, which is why tools like CC0 exist.