The French organ school formed in the first half of the 17th century. It progressed from the strict polyphonic music of Jean Titelouze (c. 1563–1633) to a unique, richly ornamented style with its own characteristic forms that made full use of the French classical organ. Instrumental in establishing this style were Louis Couperin (c. 1626–1661), who experimented with structure, registration and melodic lines, expanding the traditional polyphonic forms, and Guillaume-Gabriel Nivers (1632–1714), who established the distinct forms and styles of what was to become the French organ tradition. Characteristic forms and nomenclature French organ composers cultivated four major genres: masses, hymns, suites and noëls. Noëls are variations on Christmas carols, whereas the first three genres were all realized as collections of brief pieces in various characteristic forms. Such forms included the following: Récit: a piece in which a single voice emerges soloistically above all others by means of special registration. The latter is usually indicated in the title, i.e. in a Récit de Cromorne the solo voice would be played using the cromorne stop. Cromorne, cornet, tierce, nasard, trompette and voix humaine are the most commonly encountered solo stops. The titles of such compositions frequently omit the word "récit" and simply indicate the registration (Cromorne, Cornet, etc.) and/or the position of the solo voice. Typical combinations include the following: Dessus de Cromorne: the solo voice is in the soprano (dessus), played using the cromorne stop Tierce en taille: the solo voice is in the tenor (taille), played using the tierce stop Basse de Trompette: the solo voice is in the bass (basse), played using the trompette stop Dialogue: a piece which constantly alternates between two different registrations. A Dialogue sur les grands jeux would be alternatively played on the Grant orgue (Great) and the Positif. Fugue: usually, three- or four-voice polyphonic pieces that adhere more or less strictly to the imitative style. The designation Fugue grave indicates a piece of a serious character, whereas the Fugue gaie (or gaye) is its opposite. Rarely, four-voice fugal pieces bear the title Quatuor ("quartet"). Nicolas de Grigny cultivated five-voice fugues. Echo: phrases are played twice, quieter on the second time, giving the impression of an echo. This impression is heightened in echos that repeat only the endings of phrases. Such pieces used specially designed echo divisions. Prélude: mostly homophonic pieces in duple or quadruple meter. They are almost invariably used as introductory movements.