Roman Numeral Analysis is a way of understanding harmony and harmonic rhythm. It is a method used to define the relationships between chords and chord a group of three or more tones forming a harmony movement from one to the next. It defines chords in the context of a scale a group of pitches arranged from high to low covering one octave and tonal center.
Roman Numerals number symbols that use letters of the alphabet, I II III IV V VI VII for 1 2 3 4 5 6 7 indicate the scale degrees upon which the scale-tone chords are built. Upper case Roman Numerals indicate Major chords and lower case indicate minor or diminished.
see also: Scale-tone chords, Chord Construction, Scales and Modes, Staff Roman Numeral Chords