The source code for Tk is managed by fossil. Tk developers coordinate all changes to the Tk source code at
Release Tk 9.1a0 arises from the check-in with tag core-9.1a0.
Tk 9.1a0 continues the Tk 9.x series of releases. The Tk 9.x series do not support Tcl 8.6. The Tk 9.1 series extends the Tcl 9.0 series. To make use of Tk 9.1a0, first a Tcl 9.0 or 9.1 release must be present. As new Tk features are developed, expect them to appear in Tk 9, but not necessarily in Tk 8.