Why macOS 26 build numbers begin with 25

A question I’ve seen pop up in the Mac Admins Slack since the release of macOS Tahoe 26.x goes something like this:

Why does macOS 26 have a build number that starts with 25?

The answer has to do with Darwin, the core operating system that is the foundation for all of the following Apple operating systems:

• audioOS
• bridgeOS
• iOS
• iPadOS
• macOS
• tvOS
• visionOS
• watchOS

For more details, please see below the jump.