The typical horse skeleton has eighteen thoracic vertebrae with matching rib pairs, six lumbar vertebrae and around eighteen caudal vertebrae. Radiographic and skeletal surveys of Arabians find a raised incidence of five lumbar vertebrae, sometimes accompanied by seventeen rib pairs or sixteen tail vertebrae. The shortened back contributes to the breed's compact outline and high tail set.
The trait is variable. Studies report the reduced lumbar count in a minority of individuals rather than in all Arabians, and the same variant occurs occasionally in other breeds. It is best understood as a shift in the frequency of a normal anatomical variation, not as a fixed breed characteristic.
Popular accounts often state flatly that all Arabians have seventeen ribs and five lumbar vertebrae. That is wrong on both counts, and it matters practically: saddle fitters and veterinarians who assume a shorter back in every Arabian will misjudge individual animals. The reliable approach is radiography of the horse in front of them.