In an essay titled "Metamorphoses of Musical Form", Ligeti developed the concept of musical "permeability" according to which a musical structure is "permeable" if it allows a free choice of intervals and "impermeable" if not. Ligeti here considers Palestrina’s music as having "perhaps the lowest degree of permeability" because its handling of consonance and dissonance was the most sensitively defined of all historical styles. Ligeti saw permeability and impermeability of groups, structures, and textures in serial music as substitutes for the form-shaping function of melodic lines, motifs, and harmonies in older styles. Some textures could be layered and juxtaposed; some musical structures will mix with others seamlessly, while other structures will stand out.