A bounding wall

there is a source in the upper half-plane with a wall or barrier along the x-axis. The boundary condition at the wall is that there is no flux through the wall. The solution in the upper half-plane might also be arrived at without the wall if there were a virtual source of equal strength at the source's mirror image. This latter equivalent problem is then easier to solve.

Likewise, if a vortex was placed in the first quadrant, with walls along the positive x- and y-axes, then three virtual vortices—of equal strength, but two with opposite circulation—can be positioned appropriately in the other three quadrants so to meet the zero flux boundary conditions at the walls.

Two perpendicular bounding walls