The posterior intertransverse muscles are small muscles, best developed in cervical and lumbar regions, where they can be divided into medial and lateral slips.  

In the thoracic region anterior and posterior intertransverse muscles blend and pass between adjacent transverse processes from T10 to L1.  In the cervical region the lateral slips pass between adjacent costal lamellae and the medial slips between adjacent transverse processes from C1/C2 to C7/T1.  In the lumbar region the lateral slips pass between the transverse process of the vertebra below to the transverse process and accessory process of the vertebra above; while the medial slips pass between the accessory process of one vertebra to the mamillary process of the next from L1 to L5.  In the cervical region they are separated from the posterior intertransverse muscles by the ventral rami of the spinal nerves.

Nerve Supply
The thoracic intertransversarii and the medial slips in the cervical and lumbar regions are supplied by dorsal rami of adjacent spinal nerves: the remainder by ventral rami of adjacent spinal nerves.

Action
They produce lateral flexion to the same side in the cervical and lumbar regions, however their main function is to act as extensible ligaments stabilising adjacent vertebrae during movement.