In the roentgenological cephalometry investigators have employed several lines of reference, corresponding mostly to the planes of orientation in craniometry and cephalometry (Krogman 1951). The most common of these lines is undoubtedly the Frankfurt line, porion-orbitale, which is derived from the Frankfurt plane. This plane was defined at the anthropological congress in Frankfurt am Main, 1882. The definitions for the Frankfurt line as employed in the roentgenological studies vary, however, and do not always correspond to the anthropometrical definition. Other more or less generally used lines of reference include the NS-line, nasion-sella turcica (Brodie 1941, B jork 19471, the Bolton line, nasion-Bolton point (Broadbent 19371, nasion-basion line (Wekker 1868, Lindegdrd 1951), nasion-opisthion line (cf. Beltrami et al. 1952), and the His' line, acanthion-opisthion (His 1864, Koski 1953). These lines are clearly and singularly defined.It is our purpose in this paper to study the angular relationships of the lines of reference mentioned, special emphasis being given to the variability of the relationships.