The concept of a single-system Romulan Empire is really attractive and works within the initial episode, Balance of Terror. The name of the war, the "Earth-Romulan conflict" also makes sense, since it was one solar system vs another (albeit with some allies here and there). No need to try and explain this away in terms of the Federation.
Unfortunately, that is where the theory runs out of good ground. Every later encounter seems to refute the notion - Commodore Stocker takes a "short cut" across the NZ to reach Starbase 10, an absurb concept (in interstella terms) if the RNZ merely surrounds a single star - you are effectively cutting through a solar system! Season 3 episodes have the Enterprise actually crossing to the other side of the RNZ, in one instance getting almost instantly surrounded by enemy vessels, in another spending several hours on a hitherto uncharted planet.
I have not seen the TAS episodes recently, but from earlier discussions they don't seem the help the "single-system" theory.
Instead, I'd like to concentrate on the one piece of visual evidence we have for the Romulan Neutral Zone:
Thanks to modern HD, the markings on the map are clear. Masao Okazaki has reproduced the map in detail:
I realise that this was just a visual aid to guide the episode's action but I can't help wondering - what scale is the grid at?
The "scale" bar label shows "5000" units of something, and that each grid square is divided up into 6 sections.
5,000 x 6 = 30,000 units of something
, but what?
Throughout TOS, different measuring systems are used freely and without preference such as feet, inches, metres, miles, kilometres, light years, parsecs, even A.U.s! So the units on the scale bar really could be anything. Let's try light years:
1 Light Year = 9,460,528,400,000 kilometres
half a L.Y. = 4,730,264,200,000 kilometres
So, no joy from the metric system. But if we use miles:
1 Light Year = 5,878,499,810,000 miles
half a L.Y. = 2,939,249,905,000 miles
Half a light year is just under 3 trillion (3,000,000,000,000) miles. If each of the 6 divisions of a grid square represents 500 billion miles then each of the units that the "5000" refers to would be 100 million miles.
So, does a grid square really measure about half a light year along each side? If so, it would make for a very thin neutral zone! Also a very dense star field across the map, even assuming a three-dimensional viewpoint. For example, most of the squares have 2 or 3 stars featured. The Galaxy just ain't that full!
Maybe we should look at a different measurement unit, for example:
1 Parsec = 30,856,775,800,000 kilometres
If each of the 6 divisions of a grid square represented 5 trillion kilometers, then each of the units that the "5000" refers to would be a billion kilometers. A whole grid square would be 30 trillion (30,000,000,000,000) kilometers, not too far off a Parsec.
This makes the neutral zone about 1 Parsec deep (a comfortable distance from Romulan Space) and spreads those stars out a lot more. Plus we have a neat metric system with each unit onthe chart representing a nice round number (one billion kilometres)
Now, both Parsecs and Light Years are hopelessly Terra-centric units, since they are derived from the orbit of our own Earth around our own sun. Metres (while also Terran) are a smaller and more versatile unit that could be easily shared and used by other races. Individual species would still have their own terms of course (I doubt a Vulcan or Andorian light year is the same as a Terran one). Perhaps pan-galactic standards could be set for ease of use accross all Federation members - why not 30 trillion kilometres for a "standard" Parsec?
Then again, it is an Earth map that we see, so Earth measurements would not be too unexpected!