Types of Open Source Software Licenses: A Complete Guide

September 20, 2025

When you dive into open source, you'll quickly hear about two main camps for licenses: permissive and copyleft. Think of them as two different philosophies. Permissive licenses, like MIT and Apache, give people maximum freedom to do almost anything with your code. Copyleft licenses, like the GPL, have a key rule: if you modify the code, you have to share your changes under the same open source terms.

Your Friendly Guide to Open Source Licenses#

So, what exactly is an open source license? Forget the dense legal jargon for a second. A license is really just a rulebook for your code. It’s your way of telling the world, "Hey, I built this cool thing. Here’s how you’re allowed to use it, share it, and build on top of it."

This guide will break down the different types of licenses into those two core philosophies. We’ll look at how each approach impacts your projects, so you can confidently pick the right one. Seeing how different organizations handle this can also be super helpful; take a look at Resgrid's open source initiatives to see these ideas in action.

Why Licenses Matter#

Here’s something a lot of developers miss: if you publish code online without a license, it’s protected by default copyright law. That means people can look at it, but they have no legal right to use, change, or share it. Your code isn't truly "open source" until you explicitly grant those permissions with a license.

A license clears up any legal gray areas for you and for anyone who wants to use your software. It sets the ground rules right from the start, preventing headaches and misunderstandings down the road.

Choosing a license is not just a legal formality; it's a declaration of your project's philosophy. It defines whether your primary goal is widespread adoption or preserving long-term software freedom for all future users.

The right license can be a magnet for community contributions and even commercial use. There's a reason the MIT license is found in 92% of audited open source codebases. Its "do whatever you want" attitude makes it incredibly appealing to businesses, fostering a healthy ecosystem where your creation can really take off.

Permissive vs Copyleft: The Two Core Philosophies#

At the heart of nearly every open source license, you'll find one of two competing ideas: permissive or copyleft. If you can get your head around this one core difference, the entire licensing landscape suddenly becomes much easier to navigate.

Each philosophy takes a totally different stance on how software should be shared, tweaked, and passed along, and this choice fundamentally shapes the communities that grow around open source projects.

What Is a Permissive License?#

Think of a permissive license like a public park bench. Anyone can use it, sit on it, or even take a picture of it to build a new, fancier bench for their own backyard. The only real rule is a small plaque that says, "Donated by the Original Park Creators."

This philosophy is all about maximum freedom and adoption. It encourages businesses and individual developers to use the code with almost no strings attached, which is why it’s so popular.

These licenses usually have just one major requirement: you have to keep the original copyright and license notice intact. That's pretty much it. This minimal approach makes them a favorite in the business world, especially for developers using the best cross platform frameworks. They can easily integrate permissively licensed code into their proprietary mobile apps without being forced to open source their entire application.

Key characteristics include:

Minimal Restrictions: You can use, modify, and distribute the code freely.
Commercial Friendly: You're free to include the code in proprietary, closed-source software.
No "Viral" Effect: Your new work doesn't need to adopt the same license.

What Is a Copyleft License?#

On the other hand, a copyleft license is more like a community garden plot. You're welcome to use the tools, harvest the vegetables, and enjoy the space. But if you plant a new type of tomato using seeds from the garden, you have to share your new tomatoes—and any improved gardening tools you create—back with the community under the same open rules.

This approach ensures the garden remains a vibrant, shared resource for everyone, forever.

Copyleft licenses are built to protect and preserve software freedom for the long haul. Their central principle is reciprocity: if you use and modify the code, you must share your modifications under the same license. This is sometimes called a "viral" or "reciprocal" effect because the license's terms spread to any new software you build with it. The whole point is to guarantee that the software, and any improvements made to it, will always remain in the open source ecosystem for others to use, study, and build upon.

The core idea of copyleft is to use copyright law to ensure that every person who receives a copy of a work can use, modify, and redistribute the work and any derivative versions. It's about preserving freedom, not just granting it.

To put it all together, here’s a quick side-by-side look at how these two philosophies stack up.

Permissive vs Copyleft Licenses at a Glance#

Attribute	Permissive Licenses (e.g., MIT, Apache)	Copyleft Licenses (e.g., GPL, AGPL)
Core Principle	Maximum freedom and adoption.	Preserving freedom through sharing.
Primary Goal	Encourage widespread use in any type of project, including proprietary ones.	Ensure the software and its derivatives remain open source forever.
Main Requirement	Keep the original copyright notice.	Distribute derivative works under the same license.
Use in Commercial Software	Allowed. You can use it in a closed-source product without sharing your code.	Allowed, but you must release the source code of your modifications.
"Viral" Effect	No. Your new work can have any license you want.	Yes. The license terms apply to derivative works.

Ultimately, this fundamental split dictates how you can use, modify, and share open source code. Understanding whether a project uses a permissive license like MIT or a copyleft license like the GPL is the first and most important step in making sure you’re using it correctly.

Navigating the World of Permissive Licenses#

If copyleft is about preserving freedom with rules, permissive licenses are all about sparking freedom through simplicity. They’re designed for one thing: maximum adoption. That means placing very few restrictions on how people can use, change, or share your code.

This makes them the go-to choice for developers and companies who want their software to be used everywhere—including in commercial, closed-source products. This business-friendly vibe is why permissive licenses are the backbone of so many of the tools, frameworks, and libraries we use every day. They let you integrate code into complex projects without causing legal headaches or forcing your own proprietary code to become open source.

The MIT License: The Gold Standard of Simplicity#

When developers need a license that’s short, sweet, and gets right to the point, they almost always reach for the MIT License. It’s probably the most straightforward and popular open source license out there.

Its core principle is dead simple: do whatever you want with the code, just as long as you keep the original copyright and license notice somewhere in your work. That’s it. You can use it in your own open source projects, embed it in a commercial app you sell, or fork it into something completely unrecognizable. This incredible flexibility has made it a massive favorite across the entire software industry.

Use this when: You want to encourage the widest possible adoption of your code with almost no strings attached. It's perfect for libraries, frameworks, and utility scripts that you hope will be used everywhere, from weekend hobby projects to enterprise-level software.

This isn't just a feeling; the data backs it up. A recent analysis found the MIT License has a staggering presence in 92% of audited open source code. Its minimal requirements make it compatible with nearly any project, which is a huge reason for its dominance. You can dig into more stats on the top open source licenses and their adoption rates.

The Apache License 2.0: Corporate-Friendly and Patent-Aware#

The Apache License 2.0 is another giant in the permissive world, favored by many large-scale and corporate-backed projects like Android and Swift. It offers the same fundamental freedoms as the MIT License but adds a couple of crucial clauses that make it especially attractive for business use.

First, it includes an explicit grant of patent rights from contributors to users. In plain English, this means if someone contributes code to a project, they are also granting you a license to use any of their patents that apply to that contribution. This gives you a valuable layer of protection against patent infringement lawsuits.

Second, it's just much more detailed and explicit in its legal language. It provides clearer definitions and terms that corporate legal teams tend to appreciate.

Use this when: You're working on a substantial or corporate-backed project and are concerned about patent litigation. It’s also a great choice if you want a more legally robust license that still allows your code to be used in proprietary software.

The BSD Licenses: The Flexible Predecessors#

The BSD (Berkeley Software Distribution) licenses are a family of permissive licenses that have been around even longer than MIT. They're very similar, primarily just requiring that you give credit to the original authors in any distributed works.

The two most common flavors you'll see are:

3-Clause BSD License ("New" or "Revised"): This one includes a clause that stops people from using the names of the original contributors to endorse or promote a new product without getting permission first.
2-Clause BSD License ("Simplified" or "FreeBSD"): This version omits the non-endorsement clause, making it functionally almost identical to the MIT license.

These licenses are still common in networking stacks and operating systems. Many developers building cross-platform app development tools appreciate the flexibility that BSD-licensed components offer.

Use this when: You want a simple, permissive license but prefer the legal traditions and specific wording of the BSD family—especially if you like the non-endorsement clause found in the 3-clause version.

Understanding the Power of Copyleft Licenses#

While permissive licenses are all about granting freedom with minimal strings attached, copyleft licenses are designed to defend that freedom. They're built on a powerful, almost philosophical idea: if you build something new using my code, you have to share your creation under the same open terms.

This "share-alike" requirement is the heart of copyleft. It ensures the software and all its future variations remain a free and open resource for everyone, forever. This is why you'll often hear them called reciprocal licenses or, sometimes, "viral" licenses. The license's terms spread to any new software you build, creating a self-sustaining ecosystem of open code.

It's a powerful tool for community-driven projects that care more about the long-term freedom of their software than its use in proprietary products.

The GNU General Public License (GPL)#

When people talk about copyleft, they’re usually thinking of the GNU General Public License (GPL). It's the most famous and influential of them all, powering massive projects like the Linux kernel and WordPress. The GPL is copyleft in its purest form.

If you use any GPL-licensed code in your application, your entire application must also be licensed under the GPL. That means you're required to make your complete source code available to anyone who gets a copy of your software. This strong reciprocal rule is an intentional firewall, designed to prevent open source code from being quietly absorbed into closed-source, commercial products.

Use this when: Your number one goal is to guarantee that your software and all its derivatives stay open source, period. It's the perfect choice for foundational projects where you want to build a strong community and ensure every contribution gets given back.

The GNU Lesser General Public License (LGPL)#

But what if you love the idea of copyleft for a library, but you still want proprietary apps to be able to use it? That’s the exact problem the GNU Lesser General Public License (LGPL) solves. Think of it as a more flexible, "weak copyleft" version of the GPL.

The LGPL requires that any direct modifications to the library itself must be shared under the same LGPL terms. However, it allows other software—even closed-source commercial apps—to link to the library without being "infected" by the copyleft terms. This makes it a fantastic middle ground for libraries that need to play nice with a wide range of other software.

This flexibility is crucial in scenarios like building a component for a hybrid app development framework, where you want the component to stay open while the apps using it can remain proprietary.

The GNU Affero General Public License (AGPL)#

The original GPL was born before the rise of web apps and Software-as-a-Service (SaaS). This created a loophole: a company could modify GPL code for a web service, run it on their own servers, and never technically "distribute" the software to users. As a result, they could profit from the code without ever sharing their changes back.

The GNU Affero General Public License (AGPL) was created specifically to close this "SaaS loophole." It adds one critical rule: if you run a modified version of the software on a network and let users interact with it, you must make the complete source code of your version available to them.

Example in Action: Projects like Mastodon, the decentralized social network, use the AGPL. This ensures that anyone running their own Mastodon server must share back any improvements they make to the core software, which prevents fragmentation and keeps the entire ecosystem open.

Use this when: You're building a web application or network service and want to ensure that anyone who runs a modified public version must contribute their source code back to the community. It's the strongest form of copyleft for the modern cloud era.

How to Choose the Right License for Your Project#

Picking a license can feel overwhelming, but it really boils down to answering one big question about your project's future. Forget the legal jargon for a second and think about what you want to happen with your code.

Do you want it to be used by as many people as possible, even inside corporate, closed-source products? Or is your top priority making sure your project—and every future version of it—stays free and open for everyone, forever?

Your answer right there is the biggest step you'll take. It's the fork in the road that will lead you to the right license and define the community you build.

Key Questions to Guide Your Decision#

Before you just grab a license, take a minute to think about your project’s real purpose. The right choice will line up perfectly with your goals and set the right expectations for anyone who wants to use your code.

Just ask yourself a few simple questions:

What’s my main goal? Is it getting the code into as many hands as possible, or is it making sure every new version stays open source?
Am I cool with businesses using my code? If the answer is yes, a permissive license like MIT or Apache 2.0 is almost always the way to go. It tears down the barriers that scare off commercial users.
Is it critical that people share their changes back? If you want to build a community where every improvement benefits everyone else, a copyleft license like the GPL is your best bet.
Am I worried about patent trolls? If your project uses some clever new techniques, the Apache 2.0 license has explicit patent protections that MIT and BSD licenses don’t.

This decision tree gives you a quick visual for how these questions point you toward the right kind of open source license.

As the infographic makes clear, it all starts with how you want your code to be used. That single thought will lead you straight to either a permissive or a copyleft license, depending on what you require for commercial use and sharing modifications.

Aligning Your License with Project Needs#

There's a reason licenses like MIT and Apache are so popular in business settings. They give companies the flexibility they need and help explain why 96% of organizations are either maintaining or increasing their use of open source software.

While strong copyleft licenses are vital for protecting software freedom, they can sometimes make commercial adoption tricky. On the other hand, permissive licenses cut down on the legal friction, making it a no-brainer for businesses to build your code into their products. You can discover more insights about these trends in the State of Open Source Report.

At the end of the day, there’s no single "best" license—only the one that's best for your project. By thinking through your goals upfront, you can pick a license that doesn't just protect your work but actually helps it succeed. This kind of thoughtful planning is one of many software development best practices that great projects are built on.

Common Questions About Open Source Licensing#

Okay, so we’ve covered the big license categories. But the real world is messy, and that's where the tricky "what if" questions pop up. These are the scenarios that trip up developers all the time.

Let's walk through some of the most common uncertainties I hear about, from mixing licenses in a single project to what happens when you forget to add one at all. Getting these details right is key to keeping your project compliant and playing nice with the open source community.

Can I Mix Different Licenses in One Project?#

Yes, but this is where you have to be really careful about license compatibility. Think of it like mixing ingredients for a recipe—some combinations work great, while others can be a disaster. The rules of one license can't contradict the rules of another.

For example, you can almost always pull a permissively licensed (like MIT) library into a project that uses a copyleft license (like GPL). The stronger, more restrictive rules of the GPL will simply apply to the whole project.

But you can't go the other way. You can't just drop a GPL-licensed component into your proprietary, closed-source app without being forced to release all of your code under the GPL too. That’s a one-way street.

Navigating license compatibility is a bit like managing a privacy policy. Just as you detail data handling rules, you must ensure your code's licensing rules align to avoid conflicts. For a deeper dive into another critical policy document, you can learn more about crafting a privacy policy for mobile apps.

What Happens If I Forget to Add a License?#

This is a huge one. If you push your code to a public place like GitHub without a LICENSE file, it is not automatically public domain. Not even close.

This means that while people can see your code and even fork the repository, they have zero legal permission to actually use, change, or share it in their own projects. Without a license, your code isn't truly open source. It's basically "look, but don't touch" software, which completely defeats the purpose of sharing it in the first place.

Can I Change My Project's License Later?#

Changing a license is incredibly complicated once a project gets going. If you are the one and only person who has ever written code for the project, sure, you can change the license whenever you feel like it. You own all the copyrights.

But the moment you accept a pull request from someone else, they become a copyright holder for their contribution.

To change the license for the entire project, you would need explicit permission from every single contributor who has ever touched the code. For a project with any real traction, getting that unanimous "yes" is practically impossible. This is exactly why it's so critical to choose the right license from day one.

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