Coccyx is also a part of the weight-bearing Tripod structure.

http://en.wikipedia.org/wiki/Coccyx

Additionally, it is also a part of the weight-bearing tripod structure which acts as a support for a sitting person. When a person sits leaning forward, the ischial tuberosities and inferior rami of the ischium take most of the weight, but as the sitting person leans backward, more weight is transferred to the coccyx.