What are the different types of software license?
Four families. Permissive open source asks for attribution and nothing else. Copyleft open source requires derivative works to stay open. Source-available lets you read the code under restrictions that are not open source. Proprietary grants use under negotiated terms without source. Everything else is a variation inside one of these.
Most explanations of this list twenty licences. There are four families, and every licence you will meet sits in one of them.
The four families
software licences
│
├── permissive open source ──► MIT, BSD, Apache 2.0
│ use it for anything, keep the notice
│
├── copyleft open source ────► GPL, LGPL, AGPL, MPL
│ derivatives stay open
│
├── source-available ────────► BUSL, SSPL, Elastic, Fair Source
│ read it, restricted use, not open source
│
└── proprietary ─────────────► most commercial software
negotiated terms, usually no source The four families at a glance
| Family | What the user must do | Can a derivative be closed? | Examples |
|---|---|---|---|
| Permissive | Keep the copyright notice | Yes | MIT, BSD, Apache 2.0, ISC |
| Copyleft | Release derivatives under the same licence, with source | No, within the licence’s scope | GPL, LGPL, AGPL, MPL |
| Source-available | Follow the use restrictions, usually no competing service | Depends on the terms | BUSL, SSPL, Elastic License |
| Proprietary | Whatever the agreement says | No source is granted to begin with | EULAs, commercial subscriptions |
The first two are open source. The last two are not, and the line between them is the right to use the code for any purpose.
Permissive open source
Attribution and nothing else. Use commercially, modify, close the source of your modifications, sell the result. The only obligation is carrying the copyright notice.
MIT, BSD, and Apache 2.0. Apache adds an explicit patent grant, which is why it tends to be the choice where legal review is involved. Detailed in the MIT license and open source licenses compared.
Copyleft open source
Free to use on the condition that derivative works carry the same freedoms. Distribute something built on GPL code and that thing must be GPL too, with source.
GPL, LGPL for libraries, AGPL extending the obligation to network services, MPL applying it per file. The obligation is the point, not a side effect: it keeps derived work open rather than letting it be absorbed.
Source-available
The category that confuses everyone, because the source is right there and it still is not open source.
Open source has a definition, and central to it is the right to use the software for any purpose, including competing with the author. Source-available licences deliberately remove that right while keeping the code readable.
BUSL. Restricted commercial use for a period, typically four years, after which it converts to an open licence. A time-delayed compromise.
SSPL. Requires anyone offering the software as a service to open source their entire service stack. Written specifically to stop cloud providers reselling databases they did not build.
Elastic License and various Fair Source licences. Similar shape: read, modify, use internally, do not offer it as a competing service.
These exist because permissive licensing produced an outcome authors did not anticipate, where a hyperscaler runs your software as a managed service at scale and contributes nothing back. Whether that justifies leaving open source is genuinely argued. What is not arguable is that these licences are not open source, and calling them that is the marketing move to watch for.
┌─ WORTH CHECKING ─────────────────────────────────────────────────────┐ │ │ │ Private, finished, owned by you: $500+ each to AI labs. │ │ │ │ [ See what my repos are worth ] │ │ │ └──────────────────────────────────────────────────────────────────────┘
Proprietary
A negotiated grant of permission, usually without source, on terms covering how many people may use it, for how long, where, and with what support. Perpetual licences, subscriptions, per-seat, per-core, usage-based: all variations within this family.
Most software anyone pays for works this way, and the terms are whatever the two parties agreed. What one contains, and how to write a minimal one for your own software, is in proprietary software licences.
The other thing “license types” can mean
Search for software license types and half the results are about something else: how commercial software is sold. Perpetual, subscription, per seat, concurrent or floating, per device, usage-based, freemium, trial. Those are licensing models, and every one of them lives inside the proprietary family above. They change the price and the term, not what you are allowed to do with the code, which is usually nothing beyond running it.
If that is the question you came with, the terms a commercial licence sets and how perpetual differs from subscription are covered in proprietary software licences.
Dual licensing
Not a fifth family but a strategy worth knowing. The same code offered under two licences: a strong copyleft licence for free use, and a commercial licence for companies that want to avoid the copyleft obligation.
It only works if you own all the rights, which is why dual-licensed projects require contributor licence agreements. Accept one outside contribution without one and you can no longer relicense your own project. The full mechanism is in dual licensing.
Where your own work sits
Probably none of the above.
Code written for a client is governed by the contract, which usually either assigns ownership outright or grants a bespoke licence written by somebody’s solicitor. That is a different question, worked through in work for hire versus license and who owns the code you wrote for a client.
And private code you own carries no meaningful licence at all, because a licence governs distribution and you have not distributed anything. That turns out to be the interesting position to be in rather than the empty one, for reasons set out in your MIT-licensed repo is worth nothing.
What to read next
- Open source licenses compared, choosing between the open options
- How to add a license to a GitHub repository, the mechanics
- Work for hire versus license, for code written under contract
Common questions
- Is source-available the same as open source?
- No. Open source has a specific definition that includes the right to use the software for any purpose, including commercially and competitively. Source-available licenses like BUSL and SSPL restrict exactly that, which is their point. You can read the code; you cannot do whatever you like with it.
- What is dual licensing?
- Offering the same code under two licenses, typically a strong copyleft license for free use and a commercial license for companies that do not want the copyleft obligation. It only works if you own all the rights, which is why such projects require contributor license agreements.
- What is a proprietary license?
- A negotiated grant of permission to use software, usually without source access, on terms covering seats, term, territory, and support. Most commercial software you pay for works this way.
- Which type applies to code I wrote for a client?
- Usually none of these. Client work is governed by the contract you signed, which typically either assigns ownership outright or grants a bespoke licence. That is a different question and it is covered in the guides.
- Does the license type affect whether I can license code to AI labs?
- Only indirectly. What matters is whether the code was ever public and whether you own it. A private repository can carry any license text at all and still be licensable, because the licence in the file governs distribution you never made.
- What are the 4 types of software licenses?
- Permissive, copyleft, source-available, and proprietary. Permissive licences such as MIT only require the copyright notice to be kept. Copyleft licences such as the GPL require derivative works to stay open. Source-available licences such as BUSL show the code but restrict its use. Proprietary licences grant use on negotiated terms, usually without the source.
- Where do freeware, shareware and public domain fit?
- Freeware and shareware describe how software is distributed and priced, not what you may do with it, and legally they are usually proprietary licences. Public domain software has no licence at all because the author has given up copyright, which is what the Unlicense and CC0 attempt to do.