In the Hartree–Fock approximation the f-value trend for the 4s2S–4p2P transition of the Al sequence varies smoothly as the nuclear charge increases. The effect of the core is primarily a screening effect. However, when correlation with the core is included, 'accidental' degeneracies with other configurations at selected Z-values destroy the usually smooth trend and dips appear.The f-values for these transitions are presented and the various effects illustrated.