In 1961 Rossmann & Blow published a simple procedure for analytically combining the phase probabilities derived from various isomorphous derivatives or other phase-determining procedures [Rossmann & Blow (1961). Acta Cryst. 14, 641-647]. However, they found it necessary to make an approximation in obtaining the expression for the lack of closure (epsilon) of the phase triangle. In 1970 Hendrickson & Lattman [Hendrickson & Lattman (1970). Acta Cryst. B26, 136-143] suggested an alternative method of defining the lack of closure of the phase triangle which did not require any approximation in deriving the same analytical expression for the phase-probability function. It is now shown that it is possible to avoid the Rossmann-Blow approximation and thereby maintain the original meaning of the lack of closure as defined by Blow & Crick [(1959). Acta Cryst. 12, 794-802] and Dickerson, Kendrew & Strandberg [(1961). Acta Cryst. 14, 1188-1195].