A door or window sash comprises a main body having an insulating chamber bilaterally obtained from glazing panels whose size is larger than the size of the chamber so as to form a perimetric compartment, the first glazing panel being larger in the dimensions of its perimeter than the second glazing panel so that it projects from the bottom of the compartment to a depth which is greater than the depth of the second glazing panel; and a inner frame composed of a profiled section in the compartment, each profiled section being joined at the ends of it to corresponding contiguous profiled sections through the agency of respective joint means, each of the profiled sections defining an accessorized zone open to the outside, and the joint means comprising a reference and joining member for each pair of ends of the profiled sections. Each reference and joining member has a central body provided with two contact surfaces, at an angle to each other, for the corresponding ends of the profiled sections to be joined and in such a way as to form a corner for joining the frame; a first wing and a second wing, protruding from the angled surfaces of the central body and configured to be coupled within tubular chambers formed by the corresponding profiled sections; at least one third wing protruding laterally from one end of the central body and configured to contain, in abutment, the edges of the corner of the second glazing panel in such a way as to define an external reference point when the reference and joining member is placed in the compartment.