Roman Numerals and Chord Symbols

Calling a chord G tells you exactly one thing: its root is the pitch G. Calling it V tells you something more useful inside a key: it is the chord built on the fifth scale degree, whatever letter that happens to be. That is why theorists reach for Roman numerals. They describe a chord's job in a key, so you can compare the same progression, say I-IV-V, across any key without relearning it each time.

The numeral encodes two things at once: scale degree and quality. The degree is shown by position in the scale (I through vii), and quality is shown by case and symbols. Uppercase means major, lowercase means minor, a lowercase numeral with a degree circle (°) means diminished, and uppercase with a plus sign (+) means augmented. So in B flat major, the triad on F is V and uppercase because F major is a major triad built on the fifth degree; G minor is vi, lowercase because it's minor and sits on the sixth degree; B flat itself is I, the tonic. In A major, ii stands for B minor, the triad on the second degree, which is naturally minor in a major key.

This connects directly to inversions and seventh chords, which pile onto the same Roman numeral: a superscript 6 or 6/4 shows which chord member is in the bass, and adding a 7 extends the triad to four notes. Once you can read quality and degree from the numeral, inversion figures and seventh-chord symbols are just more detail layered on the same label.