Diatonic tetrachords are the most common and are found in most Western music. The intervals between the notes of a diatonic tetrachord are two whole steps and one half step, in that order. For example, the notes C, D, E, and F form a diatonic tetrachord.
Chromatic tetrachords are less common and are found in some types of Eastern music. The intervals between the notes of a chromatic tetrachord are two half steps and one whole step, in that order. For example, the notes C, C♯, D, and D♯ form a chromatic tetrachord.
Tetrachords are important in music because they provide the basic building blocks for melodies, harmonies, and chords. They can be used to create a variety of musical textures and effects.