In order to clarify the principles that govern the development of siliciclastic sequences and their bounding surfaces, a mathematical model of progradational basin filling was created for Atlantic-type continental margins. This paper discusses the model and its implications with respect to depositional facies, sandstone geometry, and seismic stratigraphic interpretation. Basin filling is modeled as the interaction of subsidence, change in sea level, and sediment influx. The simulations show that seismic-sequence boundaries are located, in time, near inflection points of eustatic sea-level fluctuation, where rates of fall or rise are maximized. Changes in the rate of accommodation development, both in time and space, are believed to play a dominant role in shaping the internal facies distribution, the geometry, and the nature of the bounding surfaces of depositional sequences. The pattern of coastal onlap and offshore condensed sections displayed by global-cycle charts are shown to develop in the context of smoothly fluctuating eustatic and relative sea level.