The collection (Op. 28) includes all 24 major and minor tonalities arranged in a circle of fifths, with each major key being followed by its relative minor. Contrary to custom of using this type of piece as didactic material, Chopin's cycle was intended as concert music. The "Preludes" can be seen as a microcosm of 24 categories musically expressed through basic oppositions such as mode, tempo, articulation, texture, etc. They were referred to as an extravaganza of unusual and extreme emotional states, a full reflection of the inner world, and at the same time a reflection of harmonia aeterna.