Basic Perfume Structure

The most common model used when perfumers talk about the structure and composition of a perfume is that of the picture of an isosceles triangle divided equally into three levels by two horizontal lines.
Top The portion at the top is referred to as the top notes  (first notes) or the most volatile components of of a perfume compound. This portion typically contains citrus oils, aldehydes, fruity esters.
Middle The middle section is known as the middle notes (the heart, or "coeur" from French) notes of the fragrance. Typically these will contain the floral notes of the fragrance and the last notes of the top notes plus the first notes of the bottom notes.
Bottom The bottom section is known as the bottom notes (dry-out, or lasting note), which typically contains the animal, vanilla, woody, musk and mossy notes.
 

On Paper	On Skin	.	Components
30 mins       4 hours       12 hours	30 secs       0.5 hours       2 hours		Volatile Esters   Citrus Oils   Terpenes   Herb Oils   Terpene Esters   Aromatic Esters   Terpene Alcohols   Aromatic Alcohols   Floral Oils/Absolutes   Balsams & Resinoids   Sesquiterpenes   Macrocyclic Musks

It can be a useful model to use and when looking at perfume we might talk about the balance between the top middle and bottom notes.

A fresh cologne such as 4711 typically contains a very high proportion of top notes, a much smaller middle note and an almost non-existent bottom note.

A perfume such as Diorissimo or other predominantly floral blends will tend to have a smaller top note a large heart of middle notes and a relatively smaller proportion of  bottom notes.

In the case of a heavy oriental fragrance then we tend to see a smaller top note section a good heart of middle notes but a much  larger bottom note.
 
 

Try applying the triangle yourself to a men's perfume

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When looking at the composition of a fragrance in terms of the evaporation the Middle Note is the most important and contains the central theme. This is why the French refer to it as the "coeur", the heart.
 

When we say most important this does not mean the other components are not important.  A little like a car, the engine is the heart, the coloured panels, lights and decoration the top note and the chassis and the wheels the bottom notes. The car is nothing without its heart but the engines not going very far without the support of the body or the wheels.