Limestones may be divided on the basis of their depositional environments into three categories: 1) platform, 2) basin, 3) geosynclinal. Platform limestones are thin, widespread, light colored, fossiliferous limestones which are often dolomitized. Basin limestones of the open basin type are lithologically similar to platform limestones, but display differential thickening. Barred basin limestones are typically the first stage in depositional cycles which include primary dolomites and evaporites. Basin margins are commonly marked by linear zones of reef limestones. Geosynclinal limestones deposited under shallow-water conditions are lithologically similar to platform limestones, but display great differential thickening in linear belts. Deep-water geosynclinal limestones are black, dense in texture, and siliceous. Emphasis is placed on the principle that understanding of depositional environments requires three dimensional mapping of sedimentary attributes over large areas, plus detailed study of lithologic associations.