Stereoscopic observations of high energy (~ 100 keV) photon emission from five solar flares have been made with the X-ray spectrometers aboard the ISEE-3 (International Sun Earth Explorer-3) and PVO (Pioneer Venus Orbiter) spacecraft. The observed altitude structure of the photon source and its dependence on the photon energy and time during a flare are compared with the predictions of thermal and non-thermal models of the hard X-ray source. In the case of the impulsive source, it is found that (1) the thermal model with adiabatic compression and expansion of a magnetically-confined plasma and the thin target (non-thermal) model are not consistent with the observations; (2) the thick target (non-thermal) model and the dissipative thermal model are partially in agreement with the observations; (3) the emission probably originates in many individual non-thermal sources distributed in altitude, the lower altitude sources being brighter than those at higher altitude. In the case of the gradual source, it is found that (1) models with purely coronal sources are not consistent with the observations; (2) a partial precipitation model with trapped as well as precipitating electrons is consistent with the observations.