Regions of a surface bound the materials of objects. A convex form protrudes or bulges outward as in the curved shell of an egg.
Regions of a surface bound pockets of air. Form exhibits an inward dip or hollow depression as in a spoon or bowl.
Regions of a surface bound nothing, but provide a transition between convex and concave areas. A saddle form has a compound curvature such as that of a Pringle's® chip.