* Top notes: The scents that are perceived immediately on application of a perfume. Top notes consist of small, light molecules that evaporate quickly: they form a person's initial impression of a perfume and thus are very important in the selling of a perfume. The scents of this note class are usually described as "fresh," "assertive" or "sharp." The compounds that contribute to top notes are strong in scent, very volatile, and evaporate quickly. Citrus and ginger scents are common top notes. Also called the head notes.
* Middle notes: The scent of a perfume that emerges after the top notes dissipate. The middle note compounds form the "heart" or main body of a perfume and act to mask the often unpleasant initial impression of base notes, which become more pleasant with time. Not surprisingly, the scent of middle note compounds is usually more mellow and "rounded." Scents from this note class appear anywhere from two minutes to one hour after the application of a perfume. Lavender and rose scents are typical middle notes. Also called the heart notes.
* Base notes: The scent of a perfume that appears after the departure of the middle notes. The base and middle notes together are the main theme of a perfume. Base notes bring depth and solidity to a perfume. Compounds of this class are often the fixatives used to hold and boost the strength of the lighter top and middle notes. Consisting of large, heavy molecules that evaporate slowly, compounds of this class of scents are typically rich and "deep" and are usually not perceived until 30 minutes after the application of the perfume or during the period of perfume dry-down. Some base notes can still be detectable in excess of twenty-four hours after application, particularly the animalic notes.