On an elbow X-ray the sail sign suggests an occult fracture. Its name derives from the fact that, with some imagination, it has the shape of a spinnaker (sail).[1] It rises from displacement of the fat pad around the elbow joint. Both anterior and posterior sail signs exist and both can be found on the same X-ray.
In children, a sail sign suggests a supracondylar fracture of the humerus. In adults it suggests a radial head fracture.