The Spline are a race of huge sentient space-faring creatures. They are described as fleshy spheres, encrusted with a variety of pods and orifices. The various pods contain sensory organs, weapons emplacements and other equipment, while the orifices allow for the ingress and egress of smaller ships.

The Spline evolved on a watery planet as giant, whale-like creatures. Spline technology advanced to the point where they were able to plate their organic hides with materials capable of withstanding outer space. The Spline then moved into the galaxy as living ships for hire.

Humanity first encountered the Spline during the Qax occupation of the solar system, as Spline vessels were the primary mode of Qax transport. Following the collapse of the Qax occupation, humans entered into the same sort of arrangement as the Qax had with the Spline. As the human war with the Xeelee wore on, humans employed techniques to seize control of the Spline, suppressing the conscious control heretofore exercised by the creatures.