I would like to use Roman Numeral notation for songs and do not know what to do when the piece transitions to a new key, i.e., from C to E flat major. I know this is basic, but do I try to use the Numerals for the original key, or the new key? The transition is all within the original key. An example song: We'll Be Together Again, in C major. How do you notate the transition chords to E flat?
You would have to indicate the change of key by using the letter names of the key you are going to and you also indicate whether the key is Major by the use of a capital letter and minor through the use of lower case letters.
So you can for instance have the following. (assuming we have to do with Major keys here.)
C: I IV V Eb: vi ii V
You start using the roman numerals for the chords in C major, like I,V,VI,VI etc and let's say that the C major chord chromatically changes to C minor (the vi of Eb major), you could write vi/Eb major and continue writing the roman numerals on Eb.
You could use the roman numerals:
ii/Ab major, V, I,IV.
If you are staying in the same key then bIII will be Eb major biii Eb minor. A Neapolitan 6th in C major would be Db/F bIIa I believe. I am sorry about the b representing both flats and second inversion chords here.