The number of visual binaries for which orbits have been computed now exceeds 600, but this laboriously accumulated wealth of data remains relatively sterile owing to the paucity of reliable parallaxes. Hertzsprung (1964) had this in mind when he revived the idea he first voiced in 1911 and proposed a new luminosity parameter defined as the absolute magnitude of a star reduced to unit mass without changing its density or surface brightness. The great advantage of this parameter is the fact that it is readily derived from the period and semi-major axis of a visual binary without recourse to the parallax. It therefore permits immediate use of most of the computed orbits for the construction of a luminosity-colour diagram analogous to the classical H-R diagram but entirely free of the scatter resulting from errors in the parallaxes.