A connecting device, in particular for installing at least one opening stay on a window leaf has a rail, which can be guided as far as possible in the direction of the first longitudinal axis thereof into a groove of the connecting device. The groove engages at least in sections behind the rail. The blocking of the longitudinal displaceability of the rail in the groove is guaranteed at least counter to the first longitudinal direction by a projection connected to the rail, which projection engages in a recess in the region of the groove. The projection is held in the installed position of the connecting device by a spring element in the recess. A spring element is arranged on the rail preferably in a limited movable manner. Alternatively or in addition, the connecting device can comprise a cap which holds the projection in the installed position of the connecting device indirectly and/or directly at least partially in the recess.