Notes in perfumery are descriptors of scents that can be sensed upon the application of a perfume. Notes are separated into three classes; top/head notes, middle/heart notes, and base notes; which denote groups of scents which can be sensed with respect to the time after the application of a perfume. These notes are created carefully with knowledge of the evaporation process and intended use of the perfume. The presence of one note may alter the perception of another - for instance, the presence of certain base or heart notes will alter the scent perceived when the top notes are strongest, and likewise the scent of base notes in the dry-down will often be altered depending on the smells of the heart notes.