This was prompted by another discussion on another forum. The holodeck is an idea widely and almost exclusively associated with TNG and the many stories where the holodeck placed a significant part. Indeed recently just awhile back the Star Trek Continues fan-film "Pilgrim Of Eternity" got some viewers confused and maybe even a little miffed by introducing an early form of holodeck on the Kirk era Enterprise. Some new viewers to TAS sometimes comment on the appearance of a form of holodeck in the episode "The Practical Joker," thinking the idea seems odd given the idea is so tied to TNG. But for those who might not know this the idea of a holographic recreation center was an idea going back all the way to the beginning during TOS' production. To quote from Page 190 of The Making Of Star Trek (published 1968): "The fourth major facility on the eighth deck level is the entertainment center. Certainly man of the future will require entertainment as much as we enjoy motion pictures and television today. Probably entertainment will be three-dimensional in nature and perhaps will go even further, in that you will sit in the room and the story will take place all around you. In other words, a sophisticated extension of holography. This technique will also have its effect on the traditional "mail call." Instead of receiving a letter, a man can sit in the room and, via tape, actually "see" the person sending the correspondence.As the tape is projected , the images will form in the air in front of him, so he will be able to see how his child looks, what's happening to the house, and how great his grandmother looked that day. It will be just as if he were standing there with them. Having used the "projecting unit," he can then use the "photographing unit," do a similar thing himself, and send it home..." So from this we can see they already envisioned some form of "holodeck" or "holo-room" even if they never managed to get around to showing it. Not surprising then that they used TAS (re: "the Practical Joker") to show something that might have been challenging f/x wise during TOS' production. And, of course, twenty years later the f/x of the '80s allowed them to fully realize the holodeck concept on TNG. Of course, on TNG the holodeck is also two TOS concepts rolled into one: the holographic center and the arboretum. Third season TOS (re: "Is There In Truth No Beauty" and "And The Children Shall Lead") finally showed us the beginnings of a garden like entertainment center meant to somewhat recreate the sense of being outdoors. It was also supposed to be shown in TOS' "Elaan Of Troyius, but even though the scene had actually been filmed it was edited from the final cut of the episode. This idea was conceptually elaborated on in TMP where the 8ft. filming miniature of the TMP refit shows the large windows (on the lower part of the secondary hull) of an extensive arboretum or botanical facility. Of course, on TNG the holodeck can recreate any landscape to give you the illusion of any scene or planetside locale you can imagine. Given this then it's not surprising that TAS or a fan-film could opt to show something previously conceived but unrealized on TOS. Watching TNG one could get the idea (because of the way it's presented) that the holodeck was a new form of entertainment on Picard's Enterprise, but that needn't actually be the case. In the real world many ideas can exist only in theory and discussion or even rudimentary form for years to decades or more before technology is sufficiently advanced to fully realize the concept. So there really is no reason the holodeck or some form of holographic entertainment center couldn't have existed during the TOS era.