Perfumes are usually made up of top, middle/ heart and base notes. The base notes will usually be the heaviest and strongest scent and will be smelt long after the top and middle notes have faded.
Base notes will be chosen because of their fixative properties, their strength, their scent or more often than not all three of these. The base notes will often help dictate which fragrance family a perfume sits in. There is no prescriptive scent which a base note needs to have however they will usually be one or more of the following:
Many base notes have been used for centuries and so are now rare and exotic and only used in the most exclusive and expensive scents such as Clive Christian No 1, the world's most expensive perfume. In addition many base scents are now seen as politically incorrect as they may be taken from rare animals. This has led to almost identical synthetic versions being created. Most other base notes are extracted from plant and tree resin.