The answer is when it is constructed in the form of a triangle with three distinct elements, which work together to create a beautiful fragrance, which gently changes over time.
A good story has the reader hooked. And as with all the best novels, there is often an enticing beginning, a satisfying middle and a conclusion that stays with you.In the same way when a perfumer composes a fragrance, they want it to be enjoyed over a period of time. Reacting with the warmth and oils of our skin, perfume gradually adapts and develops over a number of hours from an initial top note through to its heart and finally concludes with a lingering base.
Adding in a timeline for evaporation, the perfumer chooses their ingredients carefully considering the volatility of an oil. This is how we categorise the pyramidical notes as Top, Heart and Base.
The TOP notes give you a strong first impression but do evaporate more speedily. Just time enough to give you all but a fleeting glimpse as to how the perfume will unfold. They are the hook, with the perfumer often reaching for the light, bright oils such as bergamot or citrus.
The HEART of a fragrance is often considered the main event. It links the composition together, adding harmony and balance to make a whole, with the effect normally lasting for several hours. The heart notes are often used to categorise the perfume’s overall olfactory family, and this is mirrored by the ingredients used. For example, with floral fragrances the perfumer might blend from ingredients such as rose, geranium or jasmine.
A fragrance is rooted by its BASE, sometimes lingering on the skin for days. It is composed of those oils considered to be fixatives, which are less volatile and are slower to evaporate. Ingredients such as vetiver or oakmoss are introduced to underpin the more fragrant and fleeting notes of the earlier stages of evaporation. Their smoky, earthy notes work beautifully to add intensity to the fragrance as well as longevity.
I have to point out here that this is a starting point, a traditional way to formulate a fragrance. That the perfume pyramid is not set in stone and the perfumer is an artist who likes to play.