Abstract: A locking mechanism for windows having upper and lower sashes slidably disposed in a window frame. The mechanism has three modes, that is a locked mode, an unlocked mode and an auxiliary mode. In the locked mode at least one sash clamp mounted on a transverse rail of one sash engages a cooperating receiving element mounted to the adjacent transverse rail of the other sash, whilst a spring-biased pawl projects into a guide groove in the window frame. In the unlocked mode the sash clamp disengages the receiving element to allow the sashes to be slid relative to each other. In the auxiliary mode the spring-biased pawl is withdrawn from the guide groove to permit the sash to be moved transversely for cleaning.