If you actually write out "C major" and "C minor", then there's no need to further distinguish them via capitalization; your meaning is already clear.
So I agree with Raskolnikov in the comments that "C/c" is useful when you drop "major/minor" altogether.
However, to make it more clear, I usually use and see used "C" (or rarely "CM") for the major and "Cm" for the minor.
music.stackexchange.com/users/None/
I still see it applied for modes in chord notations, and it echoes the use of the distinction of M/m for Maximum and minimum in mathematical shorthand.
It might be a kind of political correctness: all modes are created equal, they should not be typographically discriminated.
None of them should have an initial uppercase when others have only lowercases, c minor is as respectable as A Major, and there are other scales and modes.
A related question is do you use a uppercase or a lowercase "d" with Bach's Toccata in Dorian mode BWV 538?
Most recent recordings do not respect Dorian as a mode, they only present it as a surname, like a dedication: Toccata and Fugue in D minor, BWV 538 "Dorian" Perhaps one will someday ask : Who was Dorian?
And who was she to J.-S. Bach?
music.stackexchange.com/users/48/ogerard
As suggested in my comment on the question, it seems to be a German key naming convention.
If you look at http://de.wikipedia.org/wiki/C-Moll they call it "c-Moll" (note lowercase) or "c" (note lowercase) or "Cm".
In contrast: http://de.wikipedia.org/wiki/C-Dur is called "C-Dur" (note uppercase) or "C" (note uppercase).
In other words: c-Moll vs C-Dur or c vs C or Cm vs C. That would suggest the convention is still alive in Germany (and was possibly always considered the "German style")
music.stackexchange.com/users/306/James Tauber
Most answers address the major/minor capitalization, but there's one vital reason, not yet discussed in this thread, for always capitalizing the tonic: Imagine
we're talking about the key of "a minor.
" If we don't capitalize that tonic, a certain subset of readers are destined to have some confusion (however momentary) when they read: With the appearance of a minor, the sound of a minor triad is no longer unexpected.
As such, we capitalize the tonic A, even if it's in minor, to prevent any confusion between the tonic pitch "A" and the indefinite article "a": With the appearance of A minor, the sound of a minor triad is no longer unexpected.
music.stackexchange.com/users/21766/Richard
I think it comes from the German practice as well, since German musicians were quite the standard-setters up to the 19th century.
That said, even though it's implied in the 'shorthand' notation that C represents a major and c a minor chord, I prefer to use C and Cm when writing, for absolute clarity.
The 'shorthand' notation, however, still pops here and there, when space is limited (e.g., when picking patch/combo names to save to my keyboard).
music.stackexchange.com/users/518/Gustavo Mori
In German songbooks it is still somewhat customary to write C and c for major and minor, respectively.
This is somewhat annoying to accordionists who write C for a C bass note, c for a C major chord and cm for C minor (and c+em for Cmaj7 because there are only chord buttons for the former).
So the customary oom-pah accompaniment goes C c G c C c G c on accordion, with capital letters indicating single bass notes (in a lower octave) and lowercase letters indicating chords.
music.stackexchange.com/users/42983/user42983
I was taught when I was young that minor key names should be written with the note in lowercase, as well as the word "minor". Nowadays on recordings, etc., I usually see both capitalized.  What was the original reason for this rule, and why has it (apparently) fallen out of favor?
Capitalization of key names (C Minor vs. c minor)
music.stackexchange.com
<theory><notation><key>
music.stackexchange.com/questions/877
music.stackexchange.com/users/23/Sophie Alpert
If you actually write out "C major" and "C minor", then there's no need to further distinguish them via capitalization; your meaning is already clear.   If you write out "C major" and "C minor", then there's no need to further distinguish them via capitalization; your meaning is already clear.  
If you write out "C major" and "C minor", then there's no need to further distinguish them via capitalization; your meaning is already clear.  