The sliding mode, which is caused by the switching law, is studied for the planar closed-loop switched systems. Some new conceptions are developed based on the Euclid geometry. Both the existence conditions and stability conditions of the sliding mode region around the given switching boundaries are obtained by these conceptions. And the state feedback switching laws are designed to make sure the existence and stability of the sliding mode region. As an application of the theorems, a numerical simulation is shown to validate the main results.