The base notes linger the longest on your skin -- some essential oils can last for a couple of days. While there's a wild variety of scents that make up top and heart notes (hedione high cis, anyone?), there's less diversity among base notes. This is because there aren't all that many scents that are heavy enough to stick around for as long as base notes need to.
The base notes generally comprise about 20 percent of any given fragrance. They're often from the woodsy family -- sandalwood, amber, musk and vanilla are among the most popular -- but vetiver and patchouli are common, too. As top or middle notes, these oils might be off-putting, so they need a bit of help from the heart notes to be pleasing to the nose.
You should start detecting a hint of the base note about 30 minutes after the perfume hits your skin. The "dry-down period" -- when the top and heart notes are gone -- is when the base note takes center stage. The base note's reaction with your skin is the crucial element in how the perfume ultimately ends up smelling. It's what makes a fragrance smell differently on each person, and it's also what affects your satisfaction with the perfume.
Check out the links on the next page for more information about fragrances.