A ninth chord is a type of musical chord that includes the root note, third, fifth, seventh, and ninth notes of a scale. It adds richness and complexity to music, often used in jazz, blues, and some classical compositions. The ninth can be either major or minor, depending on the context and desired sound. In notation, a ninth chord is typically represented as C9 for a C major ninth chord, which includes the notes C, E, G, B♭, and D. These chords can create tension and resolution in music, making them a popular choice for composers and musicians looking to enhance their harmonic palette.