Roman numeral analysis
In music theory, Roman numeral analysis is a type of musical analysis in which chords are represented by Roman numerals (I, II, III, IV, …). In some cases, Roman numerals denote scale degrees themselves. More commonly, however, they represent the chord whose root note is that scale degree. For instance, III denotes either the third scale degree or, more commonly, the chord built on it. Typically, uppercase Roman numerals (such as I, IV, V) are used to represent major chords, while lowercase Roman numerals (such as ii, iii, vi) are used to represent minor chords (see Major and Minor below for alternative notations). However, some music theorists use upper-case Roman numerals for all chords, regardless of chord quality.
In Western classical music in the 2000s, music students and theorists use Roman numeral analysis to analyze the harmony of a composition. In pop, rock, traditional music, and jazz and blues, Roman numerals can be used to notate the chord progression of a song independent of key. For instance, the standard twelve-bar blues progression uses the chords I (first), IV (fourth), V (fifth), sometimes written I7, IV7, V7, since they are often dominant seventh chords. In the key of C major, the first scale degree (tonic) is C, the fourth (subdominant) is F, and the fifth (dominant) is a G. So the I7, IV7, and V7 chords are C7, F7, and G7. On the other hand, in the key of A major, the I7, IV7, and V7 chords would be A7, D7, and E7. Roman numerals thus abstract chord progressions, making them independent of key, so they can easily be transposed.