Roman Numeral Notation

Most music is written in one of the twelve major scales, and the Major scale has a pragmatically-central position in a (western) musician's technique.

Because all twelve major scales have the same intervals, anything we say about the musical properties of one scale will apply to any of the others. The Roman Numeral notation lets us do this: it abstracts out the tonic note, but fixes the Major scale.

I (tonic, first) the note that names the key

♯I / ♭II (sharp first, flat second) 

II (second)

 ♯I / ♭III (sharp second / flat third) 

III (third) 

♯III / ♭IV (sharp third / flat fourth) 

IV (fourth) 

V (fifth) 

♯V / ♭VI (sharp fifth / flat sixth) 

VI (sixth) 

♯VI / ♭VII (sharp sixth / flat seventh) 

VII (seventh) leading tone to the...

I an octave above the start

Counting the semitones from the tonic, these are the same names (without the adjectives like “perfect’) as the musical intervals defined in the previous post.

All the other (equal temperament) scales can be described in terms of this one:

Natural Minor / Aeolian Mode: I-II-♭III-IV-V-♭VI-♭VII
Major Blues: I-II-♭III-III-V-VI
Whole-Tone: I-II-III-♯IV-♯V♯VI

(The ability to recite any other scale or mode in terms of "sharp this, flat that" with utter fluency is an essential skill of any academic or jazz nerd. I'm not sure how much it helps, but it sounds impressive.)