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. Nomenclature Mixture stops are typically labeled in Roman numerals with the number of ranks of pipes that they have, i.e. how many pipes sound when a single key is pressed. For example, a Mixture II contains two ranks (sounding two notes), a Mixture V contains five ranks, and so forth. The mixture is sometimes designed so that the number of ranks per note increases as one ascends the compass of the keyboard. A mixture exhibiting this trait is referred to as progressive, and is labeled with two numbers (e.g. Plein Jeu III-VII/Fourniture IV-VIII/Spakers I-V/V-X/-IV-V). If a pitch is specified (e.g. Mixture IV 2′), it refers to the pitch of the lowest-sounding rank when the lowest C on the keyboard is pressed. Sometimes a mixture is labeled with multiple numbers denoting the intervals above root pitch that it will sound. For example, Mixture 15.19.22.26 means when a note is pressed, the intervals of a 15th, a 19th, a 22nd and a 26th will sound (or 2' + 1' + 1' + '). Types of Mixtures Fourniture This is the most common type of mixture, often written simply as "Mixture". It contains octaves and fifths and is usually progressive. It is complementary to the Cymbale mixture. Cymbale A relatively high mixture containing, properly, octaves and fifths, but may include the major third. It accompanies the Fourniture to create a full chorus mixture sound. Plein Jeu Literally "full chorus" this is usually a combination of the Fourniture and Cymbale mixtures. Scharf A high pitched mixture(1',1/2'