Open source software relies on licensing frameworks that define usage rights and obligations. The two most common choices are permissive licenses like BSD and copyleft licenses like GPL. This comparison covers the facts you need to make an informed choice.

Licensing Fundamentals

Permissive licenses allow you to use, modify, and redistribute open-source code without the obligation to open source your own code. The GPL is a copyleft license, which means that it guarantees end users the freedom to run, study, share, or modify the software, but if you distribute a derivative work or modification, you must provide the source code to those recipients under the same or equivalent license terms.

This fundamental difference shapes every practical consideration below.

BSD Licenses Explained

BSD comes in two variants: BSD 2-Clause and BSD 3-Clause, with both having very minute differences to the MIT license.

BSD 2-Clause License

BSD 2-Clause requires that redistributions of source code retain the copyright notice and license text, and redistributions in binary form reproduce the same information in accompanying documentation. That is the entire scope of requirements.

BSD 3-Clause License

BSD 3-Clause adds a non-endorsement clause stating that neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. The BSD 3-Clause License was followed by the BSD 2-Clause version, sometimes known as the “Simplified BSD License”.

Historical context: The original license used on BSD Unix had four clauses, with the advertising clause (the third of four) requiring you to acknowledge use of U.C. Berkeley code in your advertising of any product using the code. Modern versions removed this advertising requirement.

BSD Core Obligations

1. Include copyright notice and license text
2. Include disclaimer (no warranties provided)
3. For BSD 3-Clause: do not use the project name for endorsement without permission

That is all. You can incorporate BSD-licensed code into proprietary software, modify it, and sell it, as long as you include the copyright notice and license text. The BSD license does not require you to share your modifications or release them under the same license.

GPL Licenses Explained

The GPL v2, initially released in 1991, is a copyleft license, meaning users must abide by strict conditions. A copyleft license requires all code modifications to the licensed software to be released under the same license.

GPL Versions

The GPL has evolved over time, with two major versions: GPLv2 and GPLv3. While both versions uphold the core principles of software freedom, GPLv3 includes additional protections against software patents and hardware restrictions.

GPLv2 says nothing explicit about patents. GPLv3 eliminates the ambiguity with Section 11 providing an explicit patent grant: each contributor gives every recipient a royalty-free, worldwide license under the contributor’s essential patent claims to make, use, sell, and otherwise run the software.

Additional protection in GPLv3: GPLv3 includes a defensive termination clause: if a licensee files a patent infringement suit against another user of the software, the suing party’s license terminates.

GPL Core Obligations

1. Disclose source code
2. Display license and copyright notices
3. State all changes made to the code
4. License derivative works under GPL (same or compatible terms)
5. Cannot sublicense under different terms

If you use a GPL library in a project that’s released, the GPL requires that you release the source code of your entire project. This is because the GPL considers the combination of the library and your code as a derived work, subject to the copyleft requirements of the license.

The Propagation Question

This is the critical difference when combining code.

BSD: The BSD license is compatible with every major copyleft license, including GPL version 2. You can use BSD code in a GPL project. You can use BSD code in a proprietary project. You can use GPL code in a BSD project (if that project becomes GPL).

GPL: If you’ve modified a program’s source code for personal or internal use, there’s no need to release its source code. However, if you make the modified program available to the public, you must also make the code public.

The GPL catches projects at distribution time, not development time.

Compatibility Matrix

GPL with Other Licenses

Apache License 2.0 is compatible with GPLv3, meaning Apache-licensed code can be incorporated into a GPLv3 project. The reverse is not true: GPLv3 code cannot be incorporated into an Apache project, because the GPL’s copyleft requirement conflicts with Apache’s policy.

Apache 2 software can be included in GPLv3 projects, because the GPLv3 license accepts Apache software into GPLv3 works. However, GPLv3 software cannot be included in Apache projects. This is one-way compatibility.

The FSF tried to increase the compatibility of GPLv3 with other licenses, but GPLv2 has no such provisions. The Free Software Foundation considers all versions of the Apache License to be incompatible with GPL versions 1 and 2.

BSD with Other Licenses

BSD licenses, being permissive, maintain high compatibility across all major licenses. This is a practical advantage for library authors targeting diverse ecosystems.

Business and Commercial Use

BSD Advantage: Proprietary Integration

If an author wants their OSS code to reach the widest possible audience, a permissive license is the best option. Companies use BSD-licensed libraries in proprietary products without legal friction.

Example: Homebrew, the macOS package manager, uses BSD 2-Clause. Go-redis uses BSD 2-Clause. These are production-critical tools with permissive licensing.

GPL Advantage: Ecosystem Protection

The primary aim of the GPL is to promote software freedom and collaboration. By requiring that modified versions are licensed under the GPL, it ensures that software remains free and open, preventing it from becoming proprietary.

Real-world example: TiVo famously used GPL-licensed Linux in its digital video recorders, leading to GPLv3’s inclusion of anti-tivoization clauses to prevent hardware manufacturers from locking down GPL software.

Use Cases

Choose BSD When

• You are building a library intended for wide adoption
• Your project will be used in both proprietary and open-source contexts
• You want minimal restrictions on downstream users
• You do not want to enforce open-source distribution of derivative works
• Patent indemnification is not a primary concern

Authors tend to choose permissive licenses like BSD because they are easy to implement, do not have many requirements, and offer flexibility.

Choose GPL When

• You want to ensure that improvements to your software remain available to the community
• You are concerned about proprietary relicensing of your work
• You want to enforce patent protections (GPLv3)
• Your project targets critical infrastructure where derivatives should remain open
• You need compatibility with other GPL-licensed projects

The GNU General Public License (GNU GPL) is considered one of the best open source licenses for developers seeking maximum freedom and collaboration. The key features that make the GNU GPL a top choice include copyleft protections: any projects that use GPL code must also release their source code under the same GPL license.

Derivative Works: What Triggers Obligations

BSD: Distributing modified BSD code triggers one obligation: include the original copyright notice and license text.

GPL: All publicly distributed modifications must be licensed under GPL. Note the word “distributed.” Internal modifications do not trigger GPL obligations.

Example: A company can modify GPL software for internal use indefinitely without open-sourcing. Once they distribute that software (to customers, on the web, or as a service), GPL obligations activate.

One exception: If copyleft should apply to web services, the Affero General Public License (AGPLv3) should apply. The AGPL adds a requirement to share modifications to source code if you make the software available to others as a service.

License Length and Complexity

BSD 2-Clause is approximately 150 words. BSD 3-Clause adds about 50 words. Both are readable in minutes.

GPL v2 is approximately 2,600 words. GPL v3 is approximately 3,300 words. Lawyers wrote these for legal precision.

This matters: To mitigate risks, permissive open source licenses like MIT, Apache 2.0, or BSD are better options for integrating with proprietary software.

Enforcement and Liability

Neither license includes liability protection beyond a disclaimer. Both licenses state the software is provided “as is” and disclaim warranties. The GPL and BSD both disclaim all implied warranties of merchantability and fitness for a particular purpose.

If your software causes damage, you are not protected by having it open source. Proper liability insurance is a separate concern.

Patent Considerations

GPLv2 says nothing explicit about patents. GPLv3 eliminates the ambiguity with Section 11 providing an explicit patent grant: each contributor gives every recipient a royalty-free, worldwide license under the contributor’s essential patent claims.

BSD licenses do not explicitly grant patent rights, though courts have implied limited patent licenses in some jurisdictions. This is unsettled law.

If you are concerned about patent protection, GPLv3 and Apache 2.0 are more explicit. BSD is riskier from a patent perspective.

Switching Licenses

You cannot easily change licenses for existing code. If you own the copyright, you can relicense future work. You cannot retroactively change the license of code you have already distributed under a different license.

Exception: Dual licensing. You can distribute code under multiple licenses simultaneously, allowing users to choose. This is common practice (BSD/GPL dual licensing, for example).

Practical Decision Framework

Compatibility Summary Table

Notes: “One-way” means Apache/MIT code can go into GPL, but GPL code cannot go into Apache/MIT projects. All permissive licenses can combine freely.

Real-World Prevalence

The Linux kernel, for example, remains under GPLv2 only. This is the most deployed GPL code globally.

Homebrew, go-redis, and Pony use BSD 2-Clause. FreeBSD uses BSD 2-Clause.

Both licenses dominate open-source ecosystems. The choice depends on your project goals, not on which is “better.”

When to Get Legal Help

• If you are combining code from multiple sources with different licenses
• If your software is used in regulated industries (medical devices, aviation, nuclear)
• If you plan to commercialize or relicense the code
• If you are concerned about patent liability
• If you are distributing GPL code and need to verify compliance

License compliance automation tools exist (FOSSA, Black Duck, SPDX) but do not replace legal review for high-stakes projects.

Key Takeaways

1. BSD is permissive: Include copyright notice, do what you want with the code
2. GPL is copyleft: Share modifications under GPL, cannot sublicense
3. Distribution triggers obligations: For GPL specifically, internal modifications are free
4. Compatibility matters: Understand what happens when you mix licenses
5. Complexity differs: BSD is simpler, GPL provides more legal precision
6. Patent protection differs: GPLv3 is explicit, BSD is unclear
7. Business integration: BSD for proprietary, GPL for ecosystem protection

The “right” license depends on whether you want your code to be reusable without restrictions (BSD) or protected from proprietary exploitation (GPL).

Reference Links

• BSD 2-Clause License
• BSD 3-Clause License
• GPL v2 Official Text
• GPL v3 Official Text
• Open Source Initiative License List
• GPL FAQ
• Apache License GPL Compatibility
• Choose a License

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