Under what license(s) are Tcl and such accessories as [Tk], [Expect],
[tcllib], ... available?
Is it possible/practical/easy/... to use Tcl in commercial products?
Is the Tcl license compatible with GPL?


The answers are ...
[[very liberal, and so forth]]

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Each author is free to use whatever license s/he wishes.  The TCT encourages people to use a BSD related license similar to the one used in Tcl/Tk/tcllib/tklib/etc. as some companies find it less restrictive.





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The license for the standard Tcl distribution is

http://purl.org/tcl/home/software/tcltk/license_terms.html
.

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[[lots more details]

It is recommended that before you begin working with an extension or application you
take a look over the license included.  If a license is not available, you may want to attempt to get some sort of license in writing so as to ensure that you don't encounter any surprises in the future.