Open source licenses explained: permissive, copyleft, and source-available

By default, code is all rights reserved. The moment you write something, copyright attaches and nobody else may legally copy, modify, or redistribute it. An open-source license is the grant that hands those rights back — and the exact wording of that grant is the whole subject. "Open source" is not one thing; it is a family of licenses that hand back different amounts, with different strings attached. This series is the practical map of that family: what each license actually permits, how they interact, and where the line between open source and merely source-available sits.

The shape of the family

Every license here answers one question — what may you do with this code, and what must you do in return? — and the answers cluster into three groups:

• Permissive (MIT, BSD, Apache 2.0): do almost anything, just keep the copyright notice. Maximum freedom for the user, minimum obligation.
• Copyleft (the GPL family): you get the same freedoms, on the condition that you pass them on — distribute a modified version and you must release your changes under the same terms.
• Source-available (SSPL, BSL): you can read the source, but the license withholds freedoms a true open-source license guarantees. These are not open source, however much they look it.

And before any of that, the word that trips everyone up: free in "free software" means freedom, not price — you can sell free software. That misconception, and the free-software-vs-open-source split behind it, is the first thing the series clears up.

My bias, stated up front

I think open source is the right default for infrastructure, and the part that matters most is not which permissive license you picked — it is whether all four of the software freedoms are actually present. That single line is what separates a license you can build a company on from one that can be revoked out from under you. I argue that case at length in Open source survives because it can fork; these learn posts are the mechanical reference underneath that opinion — the actual terms, kept neutral so you can map them yourself.

The reading path

Read in order, or jump to the one you came for:

1. What is an open-source license? — copyright, the grant, and who decides what counts as "open source"
2. Free vs open source — why "free" means freedom, not price, and the myths that follow
3. Permissive licenses: MIT, BSD, and Apache 2.0 — the do-anything licenses, and why Apache adds a patent grant
4. Copyleft and the GPL family — GPL, LGPL, AGPL, and the share-alike condition
5. Permissive vs copyleft: which license to choose — the real tradeoff and a decision framework
6. Open-source license compatibility — why you cannot freely mix licenses, and the one-way street
7. Source-available is not open source — SSPL, BSL, and the relicensing trap