The most prevalent ‘Arnold Chiari type I’ malformation typically goes unnoticed until maturity. To make a diagnosis, ‘the cerebellar tonsils' protrusion from the borders of the inner foramen magnum to their inferior most section is commonly measured (measurement is from the ophisthion to the basion). Tonsils are regarded as normal if they are located above the foramen magnum, benign tonsillarectopia is used to describe tonsil lengths of 5 mm, and Arnold Chiari type I malformation is used to describe tonsil lengths of >5 mm.