The Coccyx is a small triangular bone formed from four fused vertebrae at the base of the spine, which articulates with the sacrum to form the back section of the pelvis.
The coccyx is often called the tail bone, as it is all that remains of the tail our monkey ancestors once had.
The coccyx can sometimes become quite painful after a fall or childbirth. Also, despite the fact this small bone has no load-bearing function, many people transfer their body weight onto it when sitting or reclining in a slumped position, which puts pressure on the coccygeal nerves and makes the coccyx very painful. This pain is called coccydynia.
A coccyx cushion may reduce pressure on the tail bone whilst sitting. Perhaps preferable, is to learn to improve one's use, so that it's possible to sit in a balanced manner with the sitting bones supporting the weight of the torso, thus stopping the pressure on the tiny coccyx and in many cases, ending the pain and discomfort.