GNU/Linux is the most famous open-source operating system, with some distributions free and others open source only (http://www.gnu.org/distros/).

Microsoft Windows is a well-known example of the opposite closed-source approach. Windows is proprietary software—Microsoft owns it, restricts its use, and carefully protects its source code.

Apple’s macOS operating system comprises a hybrid approach. It contains an open-source kernel named Darwin but includes
proprietary, closed-source components as well.