Cobalt

Cobalt is the 21H2 development semester of Windows as a service, the commercial release of which is the original release of Windows 11. Work on Cobalt began in October 2020, although it did not replace Iron in the Dev Channel until December. It was later followed by Nickel in the second half of 2021.

It is the last semester to use Windows 10. Builds after 21390 are now based on the initial release of Windows 11

Wave 2 (Windows 11)
See Windows 11 (original release) § List of known builds.