Recently, I reviewed some episodes of TOS to confirm a theory. In doing so, I made an interesting discovery. For the first two years of TOS, the sound effects used for Federation spacecraft controls (particularly, buttons and switches aboard the Enterprise and her shuttlecraft) were different than those used later, in the show's final year. The regimen of sound effects (I call it a "vocabulary") used for the first two years, and for a few episodes in Year Three, were more musical and some of the sounds were longer than just beeps; some were much more expressive. In TrekCore's audio caps for "Where No Man Has Gone Before", one audio clip (from when Kirk orders the Valiant's recorder marker beamed aboard the Enterprise; starting with Scotty saying "Materializer ready, sir.") offers a significant selection of several of those sounds, played in quick succession. The second vocabulary, introduced in the show's third year, is a much simpler group of beep-like sounds, which seem to vaguely resemble touch tones of telephone technology. At some point in the show's third year, the "old" vocabulary was phased out and the "new" vocabulary was implemented. The "old", more musical vocabulary can still be heard in in the first three episodes, "Spock's Brain", "The Enterprise Incident", and "The Paradise Syndrome". Then in the fourth episode, "And the Children Shall Lead" and the fifth, "Is There is Truth No Beauty?" the "old" vocabulary is completely gone and the "new" vocabulary is used exclusively. Strangely, episode #6, "Spectre of the Gun", reverts exclusively to the "old" vocabulary. Then, oddly enough, episode #7, "Day of the Dove" starts out with the "new" vocabulary being used, but there is a brief reversion to the "old" vocabulary during the scene at Spock's science station of the bridge when he consults the Computer about his compensatory study of life units aboard ship. Then in episode #8, "For the World is Hollow and I Have Touched the Sky", the new vocabulary is exclusively used and stays in use through episodes 9 through 12. Then in episode #13, "Elaan of Troyius", the "old" vocabulary is back again exclusively for the last time. From the 14th episode for the rest of the show's third year, the "new" vocabulary is used exclusively; the "old" vocabulary is never heard again. Strange, that these sound effects changed back and forth during the show's third year, switching from episode to episode, and even switching back and forth within "Day of the Dove" briefly. [NOTE: There ordering of episodes listed above is derived directly from CBS; this listing appears to be based strictly on first-time airdates in prime time in 1968 and '69; the ordering of episodes on Memory Alpha is significantly re-arranged; still, the use of the different vocabularies is still in flux based on their listing.] I wondered about the change in sound effects, as the "old" vocabulary had obviously become so closely identified with TOS in its first two years. (The "new" vocabulary was used in association with Gary Seven's pen-like weapon in "Assignment: Earth", but not with Federation control panels.) There seems little logic to making the change at all, since neither vocabulary has any more apparent merit over the other from today's perspective. I can only suggest that since the "new" vocabulary has a vague resemblance to telephone touch-tone, it was probably chosen to replace the "old" vocabulary to sound to make the sound FX more plausible to audiences, since touch-tone was a new thing back then. (Rotary-dial phones were still widely in use throughout the 1970s; my grandmother still used an old rotary-dial desk phone right up until her passing in early 2005. The older technology, associated with more durable phones that changed little in decades, was the unquestioned standard until the breakup of the Bell System into the competitive "Baby Bells" in the early 1980s.) That's the only theory I have. I do not know of any source that provides information on this matter. Memory Alpha's wiki seems to be unaware of the change. Equally baffling was why the production kept swapping the vocabularies. It makes no sense at all. Fan films, such as Starship Exeter, New Voyages and Star Trek Continues, seem to mix the vocabularies. This seems (intentionally?) to suggest that these fan productions are assuming both vocabularies were in use aboard Federation spacecraft all along. Does anyone know of an explanation for why this happened on TOS, and why the swapping repeatedly took place?