Roman numeral analysis
In music, Roman numeral analysis uses Roman numerals to represent chords. The Roman numerals (I, II, III, IV, ...) denote scale degrees (first, second, third, fourth, ...); used to represent a chord, they denote the root note on which the chord is built. For instance, III denotes the third degree of a scale or the chord built on it. Generally, uppercase Roman numerals (such as I, IV, V) represent major chords while lowercase Roman numerals (such as i, iv, v) represent minor chords (see Major and Minor below for alternative notations); elsewhere, upper-case Roman numerals are used for all chords. In Western classical music in the 2000s, Roman numeral analysis is used by music students and music theorists to analyze the harmony of a song or piece.
In the most common day-to-day use in pop, rock, traditional music, and jazz and blues, Roman numerals notate the progression of chords in a song. For instance, the standard twelve bar blues progression is I (first), IV (fourth), V (fifth), sometimes written I7, IV7, V7, since the blues progression is often based on dominant seventh chords. In the key of C (where the notes of the scale are C, D, E, F, G, A, B), 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. In the same progression in the key of A (A, B, C♯, D, E, F♯, G♯), the I7, IV7, and V7 chords would be A7, D7, and E7. Roman numerals thus abstract chord progressions, making them independent of the key, so can easily be transposed.
...
Wikipedia