The plot of a star’s U – B colour against B – V, on the system of Johnson photometry. Stars of normal chemical composition lie on a tightly defined sequence on the diagram, if they are not reddened by interstellar absorption. Deviations from the sequence can arise for two reasons. For hot stars of spectral type O or B they are caused by interstellar reddening, which can be measured from the observed deviation. Alternatively, they indicate the ultraviolet excess in Population II stars which is related to their metal abundance. The diagram thus provides a method for measuring the metal abundances of large numbers of stars, which is useful for studying the distribution of different stellar populations in our Galaxy.