OBJECT: The purpose of this anatomical study is to identify reliable external landmarks that can be used to determine accurately the lower border of the proximal segment of the transverse sinus (TS). METHODS: The authors used 15 formalin-fixed cadaveric specimens for this project. Various anatomical structures were dissected and measurements of the distance between these structures and the proximal TS were obtained. The data collected in this study demonstrate that the inion is not always a reliable external landmark to use when determining the internal location of the very proximal TS and its drainage into the area of the torcular herophili. In addition, the authors found that the most accurate external landmark to use in reliably estimating the internal placement of the proximal TS is the point of insertion of the musculus semispinalis capitus and not the superior nuchal line. In the present study, this muscle never covered more than 5 mm of the inferior edge of the TS and was found to be a reliable anatomical structure for avoiding the medial segment of the TS. CONCLUSIONS: These findings could aid the surgeon in localizing the TS with various midline approaches to the posterior fossa and the craniocervical junction.