A Circle drawn
on a page divides it into two regions, that part of the surface
inside the circle and that part outside. When a network is drawn
on a piece of paper it will divide it into a number of regions each
bounded by arcs.
In the example there are 6 regions, 5 inside the
network and one region outside.
When a graph is drawn for an equation
such as x + y = 6, the line representing it divides the plane into
two regions which corresponds to the inequalities x + y > 6 and
x + y < 6 . Equations such as y = x2 divide the plane in a similar
way but this time the boundary line is a parabola.