Licenses are the legal DNA of software — they determine who can use it, how it can be shared, and what you must do if you change it. This guide runs through the major families of open-source and closed-source licenses, shows real-world examples, and gives resource links to the official texts.
How to read this guide
Below you’ll find succinct descriptions of common license types. Each entry includes a short explanation, typical use cases, and possible links to the official license text or other authoritative resources so you can read the exact terms.
Open-Source License Families
Copyleft (Strong)
What it means: If you distribute modified code or derivatives, the derivative must be licensed under the same copyleft license. This preserves user freedoms downstream.
Common examples: GNU General Public License (GPL) family — GPLv2, GPLv3.
Use cases: Applications, frameworks, libraries where authors want modifications to remain open-source.
Possible links:
GNU GPL v3 (official)
Affero / Network Copyleft
What it means: Similar to GPL but closes the “SaaS gap” — if the software is used over a network (SaaS), users must still be offered source for the modifications.
Common example: AGPL (Affero GPL).
Use cases: Software intended to remain open even when provided as a hosted service.
Possible links:
GNU AGPL v3 (official)
Weak Copyleft
What it means: Requires derivatives of the original files to remain under the license but allows the licensee to combine the code with other (possibly proprietary) code — rules are file- or module-scoped.
Common example: LGPL (Lesser GPL), Mozilla Public License (MPL).
Use cases: Libraries meant to be linkable by proprietary applications without forcing the whole app to be GPL.
Permissive
What it means: Very few restrictions — you can use, modify, redistribute, and relicense (even into proprietary products) as long as attribution/copyright notices are preserved.
Common examples: MIT, BSD (2-clause, 3-clause), Apache 2.0 (adds explicit patent grant).
Use cases: Libraries and small utilities where authors want maximal adoption and minimal legal friction.
Possible links:
MIT License • Apache License 2.0
Source-available & Business-friendly variants
What it means: Source code is readable, but the license imposes restrictions that are not OSI-approved open-source terms — sometimes used for “open core” or to prevent cloud providers from offering the software as a service without compensation.
Examples / notes: Various custom licenses (some vendors adopt “source-available” or “non-commercial” terms). These are not the same as OSI-approved open-source licenses.
Possible links: vendor pages and license texts (varies by project).
Creative Commons (for content, not code)
What it means: A family of licenses tailored for documentation, images, and other creative works. Not recommended for code.
Common examples: CC BY, CC BY-SA, CC0.
Possible links:
Creative Commons
Closed-Source (Proprietary) License Types
End User License Agreement (EULA)
What it means: The most common proprietary model. Grants usage rights under conditions: no source access, no modification, strict redistribution limits.
Use cases: Desktop apps, enterprise software, games.
Possible links: Vendor EULAs (Microsoft, Adobe, etc.) — vendor sites provide the actual text.
Commercial License (Perpetual / Subscription)
What it means: Paid licensing models — either perpetual (one-time) or subscription (recurring). Typically closed-source with defined support and update terms.
Use cases: Enterprise applications, SaaS premium tiers, paid developer tools.
SaaS Terms of Service
What it means: For cloud-hosted software — customers access services but do not receive source code. Legal focus is on data, uptime, liability, and permitted uses.
Use cases: Hosted CRMs, analytics platforms, collaboration tools.
Proprietary but Source-Available (Restricted)
What it means: The vendor publishes source for auditability or contribution, but the license forbids free redistribution or commercial reuse. This is different from true open source.
Use cases: Companies that want transparency without giving up commercial control.
Proprietary with Dual Licensing
What it means: The vendor offers the same code under two licenses: an open license (often copyleft) for community users, and a proprietary/commercial license for customers who need different terms (e.g., to embed the code in closed systems).
Use cases: Business models for monetizing open-source projects while allowing commercial customers to avoid copyleft obligations.
Key Concepts — Quick Reference
- Copyleft vs Permissive: Copyleft requires derivatives to stay open; permissive allows relicensing.
- Patent clauses: Apache 2.0 includes an explicit patent license; some licenses are silent and leave patent risk unaddressed.
- Distribution triggers obligations: Many licenses only require actions when code is distributed (GPL) or served over a network (AGPL).
- Compatibility matters: Combining code under different licenses can create legal friction — always check compatibility before merging projects.
Practical Scenarios
You’re building a library you want adopted widely
Recommendation: Choose a permissive license (MIT or Apache 2.0) to minimize barriers for adoption by companies and projects.
You want to ensure improvements stay open
Recommendation: Use copyleft (GPL or AGPL) to require that distributed changes remain open-source.
You run a hosted SaaS and want contributors but fear commercial rehosting
Recommendation: Consider AGPL or a business-oriented source-available license — but beware community reaction and compatibility constraints.
Official Resources & Reading
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