Abstract: A ventilation stop for limiting the length of movement of a slidable door or window sash. The ventilation stop is located in a fixed element of a window or door assembly, preferably in an interior glazing stop of a fixed sash adjacent to the slidable sash. The stop includes a recess in the glazing stop, a tab disposed in and protruding from the recess, and a hinge pin disposed in friction fit through a bore in the tab and extending into the glazing stop beyond the recess, preferably in both directions from the tab. The axes of the bore and the pin are parallel to the direction of movement of the slidable sash, permitting the tab to be turned into an interfering relationship with the slidable sash, thereby limiting the length of movement of the sash, or to be turned out of interference, thereby permitting unrestricted movement of the sash. Preferably the glazing stop is hollow, allowing for easy insertion of the pin into the tab from within the glazing stop during assembly of a window or door unit.