The equilibrium of dissociated dislocations is discussed in terms of intrinsic material properties such as the ideal stacking fault energy (ideal SFE) and the stacking fault (SF) strain. The ideal SFE is defined as the energy per unit area of an infinitely extended SF in an ideal infinite crystal. The effective SFE is defined as the energy per unit area of a terminated SF. The terminated SF is associated with a strain field. The strain field is modelled in terms of conventional and Somigliana dislocations. It is found that the effective SFE significantly overestimates the ideal SFE if the ideal SFE is low and the SF strain is large. A numerical example is given for austenitic stainless steel.