A perfect fourth is a musical interval that spans four diatonic scale degrees. For example, in the C major scale, the notes C and F form a perfect fourth. This interval is characterized by a specific frequency ratio of 4:3, which gives it a harmonious sound. In Western music, perfect fourths are commonly used in chords and melodies. They create a sense of stability and are often found in various genres, from classical to pop. Understanding perfect fourths is essential for musicians when constructing scales, harmonies, and compositions.