What is a tetrachord in music and how does it contribute to the overall structure of a musical composition?

A tetrachord in music is a series of four notes that form a scale or a segment of a scale. It contributes to the overall structure of a musical composition by providing a framework for melodies and harmonies, helping to establish the tonality and mood of the piece. Tetrachords can be combined and manipulated to create different musical phrases and motifs, adding depth and complexity to the composition.