The sacrococcygeal membrane is a tough fibrous membrane

Biological membrane

A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...

 about 10mm long which extends from the inferior tip of the sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

 to the body of the coccyx

Coccyx

The coccyx , commonly referred to as the tailbone, is the final segment of the vertebral column. Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between...

 in humans. It covers the inferior limit of the epidural space

Epidural space

In the spine, the epidural space is the outermost part of the spinal canal. It is the space within the canal lying outside the dura mater...

 and is analogous to the ligamentum flavum found at other levels in the spine

Spinal cord

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

.

It can be found at the apex of an equilateral triangle whose base is formed by the dimples overlying the sacro-iliac joints. The cornua of the sacrum may be palpated with a finger; the sacrococcygeal membrane lies between and inferior to these.