We examine stacks of several seismic phases having different sensitivities to mantle transition zone structure. When analyzed separately, underside P and S reflections (PdP and SdS) are suggestive of very different structures despite similar raypaths and data coverage. By stacking the radial component of PdP rather than the vertical PdP, we show that this difference does not result from interference from other more steeply inclined phases such as PKP and Ppdpdiff. In general, stacks of P-to-S converted phases (Pds) appear to lack evidence of a 520-km discontinuity when examined without other phases. When these phases and stacked topside P reflections (Ppdp) are analyzed jointly using a nonlinear inversion method, consistent but nonunique, seismological models emerge. These models show that a discontinuity at ~653 km depth has smaller contrasts in density and velocity than found in most previous studies. A sub-660 gradient can account for the majority of this difference. A 1.6 ¿ 0.5% P-velocity contrast and a 2.2 ¿ 0.3% density contrast at ~518 km depth without a S-velocity contrast can explain the lack of a P520s, together with robust Pp520p and S520S phases. For models parameterized with a finite thickness for each discontinuity, the 410-km discontinuity is consistently ~3 times thicker than the 660-km discontinuity.