Sigmoidal slope profiles are common on the margins of continents and intracratonic basins. We propose that these profiles consist of two genetically different segments. The lower, concave part reflects the exponential decrease of the sedimentation rate with increasing distance from the source at the shelf edge. The upper, convex part represents the gradational boundary between the shelf domain where sediment is moved by waves (and wave-induced currents) and the slope domain where sediment is moved by gravity. The boundary is narrow and the shelf edge is sharp if sedimentary base level remains stationary during progradation; the boundary becomes gradational and the shelf edge becomes rounded if storms and sea-level cycles induce significant fluctuations of base level. This model is supported by observations on Holocene sedimentary systems as well as numerical simulations with the program STRATA based on the diffusion equation for sediment dispersal.