Middle notes, often called heart notes, are the vibrant core of a fragrance, emerging after the initial burst of top notes and setting the perfume's distinct character. They bridge the gap between the fleeting top notes and the deep resonance of the base notes. 

Usually noticeable within 20 minutes to half and hour after application, heart notes can include floral tones like rose and jasmine, spicy elements like cinnamon and cardamom, or herbaceous scents like sage and lavender. Their role is crucial in transitioning from the lightness of top notes to the depth of base notes, harmonizing the entire fragrance composition. 

The differentiation between the three note categories is their volatility and duration. While top notes evaporate quickly, and base notes linger for extended periods, middle notes form the enduring core of the fragrance. In essence, middle notes are pivotal in perfumery, narrating the fragrance's story and ensuring a smooth, harmonious olfactory experience for the wearer.