The symphysis

Symphysis menti

The external surface of the mandible is marked in the median line by a faint ridge, indicating the symphysis menti, mandibular symphysis, or line of junction of the two pieces of which the bone is composed at an early period of life....

 of the external surface of the mandible divides below and encloses a triangular eminence, the mental protuberance, the base of which is depressed in the center but raised on either side to form the mental tubercle

Mental tubercle

The symphysis menti divides below and encloses a triangular eminence, the mental protuberance, the base of which is depressed in the center but raised on either side to form the mental tubercle.-External links:*...

. The size and shape of the bones making up this structure are responsible for the size and shape of a person's chin.