I've been writing software since before "open source" had a name. I watched Richard Stallman draft the GPL, saw the term "open source" coined at that Palo Alto meeting in 1998, and have spent decades explaining to well-meaning developers why "I found it on GitHub" isn't a license.
Trust me on this
I know you want to skip ahead to "which license should I pick." Five minutes on the foundations will save you hours of confusion later.
After forty-some years in this industry, I've seen every possible way to get licensing wrong. I've watched startups get cease-and-desist letters because someone didn't read the fine print. I've seen billion-dollar acquisitions nearly fall apart over license compliance issues. And I've lost count of how many times I've heard "but it's open source, that means I can do whatever I want."
It doesn't. It never did.
Why This Guide Exists¶
Every open source license is a legal agreement. When you use open source software, you're entering into a contract with people you've never met, agreeing to terms you probably haven't read. When you release your own work, you're making promises that will outlive your involvement with the project.
I wrote this guide because I got tired of watching people make the same mistakes. The concepts aren't hard, but they're not intuitive either—and the consequences of getting them wrong are real.
How to Use This Guide¶
Start with the concepts. Then explore the licenses you're likely to encounter. The case studies in Lessons Learned show what happens when people skip the fundamentals.
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Concepts
The fundamentals: what "free" really means, why copyleft exists, and the misconceptions that trip everyone up. Start here.
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Software Licenses
The licenses you'll actually encounter: MIT, Apache, GPL, and the rest. What they require, what they allow, and when to use each.
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Creative Commons
Licenses for everything that isn't code: documentation, datasets, images, research. Different tools for different work.
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Practical Guides
The "how do I actually do this" section: choosing a license, complying with licenses, and getting attribution right.
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Lessons Learned
The cautionary tales. Real incidents where licensing went wrong, and what we can learn from the fallout.
Quick Reference¶
In a hurry? The quick comparison table gives you the essentials at a glance.
Confused by a term? Check the glossary, or just hover over highlighted terms—they'll explain themselves.
The obligatory disclaimer
I'm a developer, not a lawyer. This guide reflects decades of practical experience, but it's not legal advice. For specific legal questions about your situation, talk to an actual attorney. They're not as scary as you think, and they've saved more than a few projects I've worked on. There's only so much the folks at r/legaladvice can do, and if you're hitting that limit, it's time to find real legal advice.