An isoelectronic comparison of spectra in the P I, Si I and Al I sequences has been made for the elements K, Ca, Sc and Ti. In the course of this study we have also identified some lines in the spectra of adjacent elements in the same sequences, for example S IV, Ar IV, Ar V and Ar VI. In the P I and Si I sequences the study is restricted to transitions to the ground configurations 3s²3p² and 3s²3p³. In the Al I like spectra we have identified transitions between the configurations 3s²3p, 3s3p², 3p³, 3s²nd, 3s²nf, 3s3p3d and 3s3p4s, where the nd and nf series have been traced to n = 6, and for nf in Ca VIII to n = 7.