The fourth interval, also known as a perfect fourth, is considered perfect because of its consonant and stable sound. It spans three whole tones (five half steps) and has a bright and clear quality. This interval is often found in chords and progressions, such as the I-IV-V progression in major keys, which creates a sense of resolution and stability.
The fifth interval, also known as a perfect fifth, is also considered perfect due to its consonant and harmonious sound. It spans four whole tones (seven half steps) and has a clear and resonant quality. The perfect fifth is one of the most fundamental and important intervals in music, as it forms the basis of many chords and scales.
Both the perfect fourth and perfect fifth are considered perfect because they are consonant intervals, meaning they sound pleasing and stable when played together. They are also used extensively in music theory and composition, serving as important building blocks for harmonies, melodies, and chord progressions.
In summary, the fourth and fifth chords are considered perfect because of their consonant and pleasing sound, their use in fundamental musical structures, and their role in creating a sense of resolution and stability in music.