When an interpreter is created Tcl initializes this variable to hold a string giving the current patch level for Tcl, such as 8.4.16 for Tcl 8.4 with the first sixteen official patches, or 8.5b3 for the third beta release of Tcl 8.5. The value of this variable is returned by the [info patchlevel] command. (From "tclvars" man page)