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.

The Darwin operating system has its own version number which is separate from the version number of the other Apple operating systems. In the case of macOS, Apple includes Darwin’s version number in the macOS build number. For example, the build number for macOS Tahoe 26.2.0 is the following:

25C56

The 25 part refers to the major version number for the current Darwin release. Beginning in 2011, the major version numbers of the Darwin operating system have coincided with the last two digits of the year it was released:

The current Darwin operating system release is 25.0.0 as of macOS Tahoe 26.2.0, so macOS Tahoe build numbers start with 25. Unless Apple decides to change Darwin’s version numbering system, this will mean that future macOS releases will continue to have build numbers that will start with a two digit number that is one number less than the macOS major version number.