[en] This paper reports on perovskite KNbO₃ thin films heteroepitaxially deposited onto (100) MgO substrates. Twin domains with tetrahedral shape were typically observed. These tetrahedrons were bounded by three [211] twin planes. High resolution transmission electron microscopy (HRTEM) was employed to examine these [211] twin boundaries. Surface steps of [110] left-angle 110 right-angle type generated by dislocation slip were present on the MgO substrate. The tetrahedral twin domains originate on surface steps, then grow with a stacking fault relationship to the matrix. The strain fields of dislocations near the stacking faults slightly rotate the tetrahedral twin nuclei. This small degree of misalignment between the matrix and the twin domain result in some of the twin boundaries having amorphous boundary regions rather than coherent interfaces. Order- disorder antiphase domains (APD's) were directly imaged with HRTEM. Ultra small APD's ranging from 10 to 30 atomic spacings were observed. The origin of these APD's was either the surface steps of [100] left-angle 110 right-angle type on MgO substrates or the random nucleation of anions at either of two equivalent sites on the MgO substrate