Three distinct morphologies of curved (curved, facetted, and corrugated) inversion domain boundaries (IDB's), observed in aluminum nitride, have been investigated using conventional transmission electron microscopy, convergent beam electron diffraction, high-resolution transmission electron microscopy, analytical electron microscopy, and atomistic computer simulations. The interfacial structure and chemistry of the curved and facetted defects have been studied, and based upon the experimental evidence, a single model has been proposed for the curved IDB which is consistent with all three observed morphologies. The interface model comprises a continuous nitrogen sublattice, with the aluminum sublattice being displaced across a {1011} plane, and having a displacement vector R = 0.23〈0001〉. This displacement translates the aluminum sublattice from upwardly pointing to downwardly pointing tetrahedral sites, or vice versa, in the wurtzite structure. The measured value of the displacement vector is between 0.05〈0001〉 and 0.43〈0001〉; the variation is believed to be due to local changes in chemistry. This is supported by atomistic calculations which indicate that the interface is most stable when both aluminum vacancies and oxygen ions are present at the interface, and that the interface energy is independent of displacement vector in the range of 0.05〈0001〉 to 0.35〈0001〉. The curved IDB's form as a result of nonstoichiometry within the crystal. The choice of curved IDB morphology is believed to be controlled by local changes in chemistry, nonstoichiometry at the interface, and proximity to other planar IDB's (the last reason is explained in Part III). A number of possible formation mechanisms are discussed for both planar and curved IDB's. The Burgers vector for the dislocation present at the intersection of the planar and curved IDB's was determined to be b = 1/3〈1010〉 + t〈0001〉, where tmeas = 0.157 and tcalc = 0.164.