This listing is for one pale yellow valve without the offset mount membrane. The yellow valve fits snugly onto the base of the breast shield connector, and typically holds the white membrane in place.The yellow valve should be removed from the breast shield connector after each pump use. The white membrane should be removed from the yellow valve and both parts cleaned. If these parts are not cleaned, milk can be trapped between them leaving the white membrane sticky. This will prevent the white membrane from moving properly during pumping and decrease effectiveness of the pump.