This is the release 4.1.2 of Itcl. It is intended to be script compatible with Itcl 4.0.* and Itcl 3.4.* . It very likely presents the same public C interface as Itcl 4.0.* . It includes incompatible changes to internal structs when compared with Itcl 4.0.* . Unfortunately, the extension Itk 4.0.* intrudes in those internals and will notice and break in the presence of Itcl 4.1.* . When you upgrade to Itcl 4.1 , you must also upgrade to Itk 4.1 . It is possible you will find other extensions and applications repeating Itk's error. Notes of past releases follow below ----------------------------------- The difference to 4.0.*: in this release there are only bug fixes from SF and the fossil bug tracker for itcl. This is the first stable release of Itcl 4.0. It is a new major release of Itcl. The difference to 4.0b7: Tighter control on the set of exported functions. The difference to 4.0b6: Updated TEA system and related build system changes. The difference to 4.0b5: in this release there are only bug fixes from SF tracker and updates for using Tcl 8.6 version from fossil repo trunk The difference to 4.0b4: in this release there are only bug fixes from SF tracker and updates to TEA 3.9. The difference to 4.0b3: in this release there are only bug fixes from SF tracker and some fixes to run on OSX and Windows platform. There is no known incompatibility.