It is possible to define a specific minimum isoperimetric quotient, proportional to the ratio between the area and the square of the perimeter of any given congressional voting district. Although technologies presently exist to define districts in this manner, there are no rules in place mandating their use, and no national movement to implement such a policy. One problem with the simplest version of this rule is that it would prevent incorporation of jagged natural boundaries, such as rivers or mountains; when such boundaries are required, such as at the edge of a state, certain districts may not be able to meet the required minima. One way of avoiding this problem is to allow districts which share a border with a state border to replace that border with a polygon or semi-circle enclosing the state boundary as a kind of virtual boundary definition, but using the actual perimeter of the district whenever this occurs inside the state boundaries. Enforcing a minimum isoperimetric quotient would encourage districts with a high ratio between area and perimeter.