Twin laws by (reticular) pseudo-merohedry are investigated for which the obliquity δ vanishes for an appropriate value of the axial ratio c/a. For arbitrary values of c/a, it is shown that δ is related to the deformation parameter ∊ introduced by Bonnet & Durand [Philos. Mag. (1975>), 32, 997–1006]. Furthermore, tanδ is a product of two factors: one describes the dependency on the lattice parameters, the other the effect of the lattice misorientation.