A ninth chord is a type of musical chord that includes five notes: the root, third, fifth, seventh, and ninth. It is often used in jazz and popular music to create a richer, more complex sound. The ninth is typically added to a seventh chord, which already contains four notes. In music theory, ninth chords can be classified into different types, such as major, minor, and dominant. Each type has a unique sound and function within a piece of music. Musicians often use ninth chords to add color and tension, enhancing the emotional impact of a composition.