Perfume is a mixture of substances with different degrees of volatility. Fresh notes are predominantly light, very volatile substances which characterise the first phase of the fragrance's life on the skin, the top note. Substances with less volatility produce the bouquet, the body, or in other words the middle note of the perfume. Substances with very low volatility and longer adhesion characteristics produce the foundation.
A good perfume is built very carefully, with the objective that similar smelling substances will be effective throughout all three phases of the fragrance life, making one continuous impression. As the moment of the first try, the user is aware of only the initial smell - the top note. With good perfumes, there should be little difference between this and the succeeding phases. With not so good ones, the phases can be disturbingly different.