The shape, origin, termination and number of the main arteries of the vertebrobasilar system have been analysed in 30 anatomic specimens by dissection of the injected vessels and by radiography. Five projections are described for identifying the different regions of the brain stem and cerebellum through the appearance of the arteries. A technique is proposed for obtaining a 'true' a.p. view of the vessels in vertebral angiography in patients.