Notes in perfumery.

Fragrance pyramid.
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 smells that 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. However, each class of notes is not as easily sensed at a certain time they can alter the smells of another note class. For instance, the presence of a 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 be often be altered depending on the smells of the heart notes.

The idea of notes are used primarily for the marketing of fine fragrances. They are used less occasionally by perfumers to roughly describe scents or the perfumery process to layman.

Top notes
Perceived immediately upon 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.

Although not as saliently perceived, the heart and base-notes contribute much to the scent in the top notes.
Middle notes
The scent of a perfume that emerges just prior to when the top notes dissipate. The middle note compounds form the "heart" or main body of a perfume and emerge in the middle of the perfume's dispersion process. They serve 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. They are also called the "heart notes".
Base notes
The scent of a perfume that appears close to 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.

The perfume of labdanum belong in Base notes.