In this paper, a link between poly-quadratic stability and stability of arbitrary switching systems is established. Polyquadratic stability aims to check asymptotic stability of a polytopic system by means of polytopic quadratic Lyapunov functions. The necessary and sufficient condition of poly-quadratic stability proposed in Daafouz and Bernussou (2001) is shown to be immediately applicable to a class of switched control and observer design problems. Chaos synchronization for which the transmitter is described by a piecewise linear map is presented as an application.