Left out in this description of 
figure-ground is any mention of 
borders, the assignment of 
which is central to shape 
perception. At any given time, 
we assign the common border 
between regions to one region 
or the other, with the 
consequence that the region to 
which the border is assigned 
takes on a particular perceptual 
shape. When the assignment is 
reversed, the shape perceived 
is dramatically different. For 
example, in the familiar faces-
or-vase pattern, we assign the 
borders to either the two 
profiles or to the sides of the 
vase. A border, that is, cannot 
belong to both a profile and the 
vase at the same time.  When 
the border is assigned to the 
faces rather than the vase, the 
figure’s entire appearance is 
quite different: what appeared 
concave in the borders of the 
profiles become convex in the 
edge of the vase, and so on.