The formal stimuli for vers libre were vers libéré (French verse of the late 19th century that liberated itself from classical rules of versification whilst observing the principle of isosyllabism and regular patterned rhyme) and vers libre Classique (a minor French genre of the 17th and 18th century which conformed to classic concepts, but in which lines of different length were irregularly and unpredictably combined) and vers Populaire (versification derived from oral aspects of popular song). Remy de Gourmont's Livre des Masques gave definition to the whole vers libre movement; he notes that there should arise, at regular intervals, a full and complete line, which reassures the ear and guides the rhythm.