The flap of skin under a person's tongue is called the frenulum. The frenulum is a mucous membrane that is in the shape of a "V."
Babies often have a condition called tongue-tie, or ankyloglossia, that restricts the range of the tongue's motion with a shortened frenulum. This is not a serious condition, but it can make breastfeeding tough on a mother and newborn. The shortened shape of the frenulum makes it difficult for the baby to latch on and stick his or her tongue out. Treatment of this condition involves a short surgical procedure shortly after birth. The tongue and frenulum will be fully developed at 4 weeks of age.