Enamel, the clear outer layer of the tooth above the gum line, is the hardest substance in the human body. The visible portion of the tooth is called the crown. Projections on the top of each crown, used primarily for chewing and grinding, are called cusps. The portion of the tooth that lies beneath the gum line is the root. The periodontal ligament anchors the tooth in place with small elastic fibers that connect the cementum in the root to a special socket in the jawbone called the alveolus.