Yes. For multithreaded code, instead of deciding where/how to put interpreter locks (like Python's infamous GIL (global interpreter lock)) to manage low-level sanity wrt sharing data, Tcl punted and lets each interpreter run unfettered.

This is called apartment model [0] threading.

[0] https://msdn.microsoft.com/en-us/library/aa261361(v=vs.60).a... (this is a Microsoft VB article, with nothing to do about Tcl specifically, but does nicely explain the apartment threading model)

Edit: s/let's/lets/