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According to an interview with John Jefferies, one of the set designers, stated in the TOS Season 2 DVD special feature, "Designing the Final Frontier", The plumbing and pipes in the Enterprise all had color codes and some form of nomenclature on it. That would be a series of letters and numbers and something that looked like you could tell what this pipe was on this side of the wall, as it came out of the wall. And GNDN with a number behind it and then a color code became very common throughout the Enterprise. GNDN means 'Goes nowhere, does nothing'." This became a well-known enough prank that when the Star Trek: Deep Space Nine crew re-built some of the old Enterprise sets for the 30th anniversary tribute, "Trials and Tribble-ations", they made sure to include a GNDN pipe.