Fragrances typically have between 30 and 100 different components, or "notes". Some components are more dominant in the first 15 minutes and are called the "top notes". Others become more apparent after an hour or so and are called the "middle notes" or "heart notes". Finally, the "base notes" are what remain after three or four hours. I believe this changing nature adds to the interest and appeal of modern fragrances, and maintaining its harmony is one of the most challenging aspects of perfumery.