The middle, or heart, note makes up the core of a fragrance -- it usually comprises around half of the oil in the bottle. The heart notes are generally composed of more robust oils because they need to hold up longer on the skin. But because they're the main fragrance, they also have to win you over -- this has to be a scent you'd be willing to live with.
The heart notes usually evaporate within two to four hours, leaving the base note to react with your skin. Heart notes act as kind of a buffer for the base notes, which might not smell quite as pleasant on their own. Even if a fragrance isn't officially recognized as a "floral" or describes itself as such, heart notes are overwhelmingly composed of floral oils. Lemongrass, geranium, neroli and jasmine are common ingredients.
Because the heart notes have the responsibility of being the perfume's backbone, they're often more complex than the top notes. CK One by Calvin Klein, the ubiquitous '90s unisex fragrance, lists hedione high cis (a jasmine derivative), violet, rose, nutmeg and "green tree accord" as its heart notes. Clinique's Happy, another best-seller from that era, has heart notes of boysenberry bush flower, morning dew orchid and melati blossom.
After the top notes and middle notes have disappeared into thin air, the base notes hang in there ...