The aroma of an essential oil generally is broken down into three major components: “top”, “middle”, and “base” notes.
Aromatherapy oil top notes are those that are light and fresh in aroma and the first that the nose will discern. They predominate in oils that evaporate more quickly, such as grapefruit and peppermint.
The middle notes provide the heart, and often the bulk, of the fragrance, and are for this reason to be found in almost all essential oils.
The base notes are heavy and rich in character, and are the last to emerge from the scent. They give to oils such as benzoin and patchouli a fixative capacity, as they evaporate slowly and have a strong tenacity.