Equatorial inertial instability simulated in middle atmosphere models is characterized by vertical gridscale structures in the zonal-mean winds and temperature. This paper will examine the three-dimensional structure of the instability. It will be shown that the instability has a zonal wavenumber dependence that is selected by the winter-hemispheric planetary waves impinging upon the zero-wind line. The breakdown of the unstable region will be examined, along with the effects on the zonal-mean state. Modification of the potential vorticity gradient on the summer side of the equator by inertial instability creates a region of barotropically unstable flow that leads subsequently to the excitation of two-day waves. It will be demonstrated that the inertial instability also generates small-scale travelling waves. Examination of the dispersion relation suggests that these are gravity waves.