The 1×1 strategy is sufficient for detecting boundary errors and the Nx1 strategy is effective foretermining exactly what type of boundary error is present, where N is the dimension of the input space.  The 1×1 strategy tests two points for each boundary inequality, one on the boundary and one off the boundary.  The two points are chosen as close as possible to one another to ensure that domain shift errors are properly detected. If two domainsshare a boundary, then the off point is always in the domain that is open with respect to that boundary.