Isothermal sections of the phase diagrams FeCo M As M′ , and CoNi M As M′ , have been redetermined. Because of the easy substitution Fe-Co and Co-Ni the marginal phases extend considerably into the three-component range and concomitantly change the axial ratio of their cells. The phase Fe1-NCo N As, in particular, changes its axial ratio from the long subtype of the MnP type in FeAs continuously to the short subtype in CoAs. An interpretation of the experimental results becomes possible when it is assumed that following Ekman's rule the A atoms iron, cobalt, nickel do not essentially contribute to the valence electron correlation with cellb, and that the d electrons of the A atoms form a correlation of their own with celle, not comprising the d electrons of arsenic. A phase becomes stable when there is favourable commensurability between the cellsb, e, c of the different spatial correlations of the electrons and the cella of the structure From the binding interpretation emerge new arguments for the long and the short subtype of the MnP type, for the magnetic maturation moments of the elements iron, cobalt, nickel, and for other phenomena.