You older guys may remember that in the days when the "Big Three" predominated the U.S. airwaves, it was forbidden to even mention a competitor's network on a talk or variety show. That is why, for example, you might have heard a Carson or other host say that a guest's new show "premieres this Thursday at 9 pm on 'another network.'" I wonder, was this taboo set in stone (contractually) or just a time-honored convention? And when did the mentioning of the competition's acronym cease to be verboten?