(1) Attached or relief sculpture. (1.1) Bas-relief sculpture is believed to be the oldest sculptural manifestation since prehistoric cave art. (1.2) The relief sculpture is attached to the surface but has a protruding part creating a sense of 3D.
(2) Freestanding or round sculptures are later; now the work can be examined from all angles. Another, much later, step was to display round sculptures without plinths.