A mixture is an organ stop, usually of principal tone quality, that contains multiple ranks of pipes including at least one mutation stop. It is designed to be drawn with a combination of stops that forms a complete chorus (for example, principals of 8′, 4′, and 2′ pitches). The mixture sounds the upper harmonics of each note of the keyboard, adding a chord to each note played. The individual pitches in the mixture are not distinguished by the listener; rather, they reinforce the fundamental pitches of the chorus, adding volume and brilliance to the sound. Because pipes playing upper harmonics produce their own set of harmonic overtones, an element of harmonic dissonance is introduced, giving mixtures their characteristic tonal texture as they enrich the ensemble. Historically, the mixture descends from the medieval Blockwerk concept, an organ in which there were no stops and all the ranks sounded simultaneously.