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This paper addresses the behavior of the Common Format Equation (CFE)constraint factor Ω*, for C(T), SE(B), and M(T) specimens. Values of Ω* are obtained fromnumerical data for these three geometries as a function of the size parameter B/b, i.e., thethickness-to-ligament ratio. The results indicate a geometry- and size-dependence of theconstraint factor in the range of B/b employed, roughly from 0.15 to little less than 4.0. TheC(T) and SE(B) specimens show a smooth transition of their Ω* values from plane strain toplane stress in the whole B/b range. In the same range, the M(T) specimens show muchlower Ω* values. One clear consequence for the M(T) geometry is that it may never reach aplane strain state, as defined by the CFE, when compared to C(T) and SE(B) specimens.