This is a rule of thumb that helps create essential oil blends that have complexity and longevity. It says that 30% of your blend should be top notes, which are light, fresh, and uplifting scents that are easily detected but that evaporate quickly. 50% should be middle notes that provide balance and depth, such as lavender, chamomile, or rosemary. And 20% should be base notes, like sandalwood, cedarwood, or patchouli, that give a blend its staying power.