What does the functional chord symbol 'I' represent in a major key?

The functional chord symbol 'I' represents the tonic chord in a major key, which is built on the first scale degree. In Western music theory, the tonic is the home chord and serves as the main point of rest and resolution in a piece of music. In a major key, the tonic chord consists of the root note, the major third, and the perfect fifth. This combination creates a stable sound that forms the foundation for much of the harmonic structure within a composition.

The importance of the tonic is reflected in its role in establishing the key of the piece, defining its tonal center, and providing a sense of closure at the end of musical phrases. The tonic chord is often the starting and ending point for musical compositions, emphasizing its significance.

In contrast, the other options represent different functions within the key. The subdominant chord, which is built on the fourth scale degree, acts as a precursor to the dominant and does not serve the same foundational role as the tonic. The dominant chord, built on the fifth scale degree, introduces tension that typically resolves back to the tonic. Finally, a dominant 7th chord includes an additional note that adds complexity and tension but still does not serve the same primary function as the tonic chord itself.