The basic equations are presented for the gravity effect of a horizontal slab with density varying linearly between limits. A series of type curves and interpretation nomograms are presented showing the characteristics of the gradational model. A notable characteristic is the horizontal gravity gradient which is nearly constant over most of the width of the variable density zone. Maximum slope and curvature of the gravity effect vary directly with the density change, depth, and thickness of the slab and inversely with the width of the variable density zone. Practically identical gravity effects are sometimes produced by step models with different dimensions and density contrasts. However, if the variable density zone is sufficiently wide, the gravity effect cannot be approximated by a simple step model.