MRI holds great promise for diagnosing Parkinson's disease, based on the disappearance of the swallow tail sign in T2*-weighted images, which past research assumed to be nigrosome 1. We studied the sign's anatomical underpinning, combining ultra-high field MRI in vivo and postmortem, 3D-reconstructed microscopy, and immunohistochemistry. Based on block-face images and calbindin-D28K immunohistochemistry, we constructed a 3D nigrosome atlas. We show that nigrosome 1 extends beyond the swallow tail sign by co-registering this atlas to in vivo MRI. As the swallow tail sign only partially overlaps with but is not identical to nigrosome 1, its interpretation needs to be revised.