In the perfume industry, fragrances are developed in three layers: the top notes, heart notes, and base notes. The top notes are comprised of the aromatic oils you notice first, and they last the shortest amount of time – generally, just a couple of hours. Typically, the top notes are the lightest scents, such as green or herbal oils. The heart notes are the middle oils in the fragrance that start to become more apparent after two or more hours of wear. These are the body of the fragrance, and they usually are more potent than the top notes. Often, you’ll find aquatic, leather, or mossy oils in the heart of the cologne. The base notes aren’t really detectable until several hours have passed, and they usually only become apparent after the top notes have faded away, which is why the smell of a cologne can change so much as the day goes by. Base notes are usually rich, potent oils, such as woody or oriental scents.