: coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all apes,[1] and analogous structures in certain other mammals such as horses.

In tailless primates (e.g. humans and other great apes) since Nacholapithecus (a Miocene hominoid),[2][3] the coccyx is the remnant of a vestigial tail.

In each of the first three segments may be traced a rudimentary body and articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone.

The first segment is the largest; it resembles the lowest sacral vertebra, and often exists as a separate piece; the remaining ones diminish in size rostrally.

The lateral borders are thin and exhibit a series of small bony protrusions, which represent the transverse processes of the coccygeal vertebrae.

[7][8] The anterior side of the coccyx has attachments to the levator ani muscle, coccygeus, iliococcygeus, and pubococcygeus, anococcygeal raphe.

[10] Some fibers of the sacrospinous and sacrotuberous ligaments (arising from the spine of the ischium and the ischial tuberosity respectively) also attach to the coccyx.

It is doubtful that the coccyx attachments are important to the well-being of humans, given the large number of coccygectomy procedures performed annually to treat coccydynia.

Injuring the coccyx can give rise to a painful condition called coccydynia and one or more of the bones or the connections thereof may be broken, fractured tailbone.

[24] The 16th/17th century French anatomist Jean Riolan the Younger gives a rather hilarious etymological explanation, as he writes: quia crepitus, qui per sedimentum exeunt, ad is os allisi, cuculi vocis similitudinem effingunt[20] (because the sound of the farts that leave the anus and dash against this bone, shows a likeness to the call of the cuckoo).

This Latin expression might be the source of the English, French language, German and Dutch terms tailbone, l'os de la queue,[25] Schwanzbein[21][25] and staartbeen.