From the original pitch, they were to explore habitable planets with (E)arth-(M)ars, or E-M, conditions, so it was probably originally "em"-class planets.
, I listed what the third revision of the Star Trek Writers/Directors Guide
had to say about Class M, and I described why in the 1970's I assumed -- with others -- that the "M" in "Class M" stood for "Mars".
Here's what the original pitch for Star Trek
said, created by Gene Roddenberry, first draft, dated March 11, 1964:
Original Star Trek Pitch wrote:
The "Parallel Worlds" concept is the key ...
... to the STAR TREK format. It means simply that our stories deal with plant and animal life, plus people, quite similar to that on earth. Social evolution will also have interesting points of similarity with ours. There will be differences, of course, ranging from the subtle to the boldly dramatic, out of which comes much of our color and excitement. (And, of course, none of this prevents an occasional "far out" tale thrown in for surprise and change of pace.)
IV. Nature and duration of command:
Galaxy exploration and Class M investigation: 5 years
VII. Consistent with the equipment and limitations of your cruiser class vessel, you will confine your landings and contacts to planets approximating earth-Mars conditions, life, and social orders.
Some format and budget considerations ...
SETS. Our format is tailored to practical production and cost factors. Use of stage sets, backlot and other locations are ximplified [sic] by Captain April's "Class M" orders. And our own "Parallel Worlds" concept. The majority of story premises listed can be accomplished on such common studio backlot locales and sets such as Early 1900 Street, Oriental Village, Cowtown, Border Fort, Victorian Drawing Room, Forest and Streamside.
(If I missed any references to "Class M" or "Mars", it's because I had to search without the aid of a computer. I omitted some text regarding the "Parallel Worlds" concept. Any errors entering this text are my own.)
As with the Star Trek Writers/Directors Guide,
the original pitch said, in so many words, that "Class M" stands for "earth-Mars conditions" [sic]. However, the spelling of "Class M" in the pitch was always with the capital letter "M". If it was ever spelled "em"-class,
as cleverly suggested by Darkwing
, then it was spelled that way before the first pitch was written. It's good to know that the phrase "earth-Mars conditions" goes all the way back to the original pitch.