This product is made by Maymom to fit spectra S1/S2 pumps in addition to spectra 9 and spectra M pumps. It is known that using the correct size of flange (breastshield) in breastmilk pumping is the most important factor in achieving comfort and efficiency. Every mother was created differently. One or two sizes can't fit all. That is why Maymom develops more than 12 sizes of flanges spanning from 13 mm up to 36 mm to fit the average mothers and those mothers who need special sizes (e.g. 13, 15, 17, 36 mm). For pumping comfort, measure your nipple diameter first (do it after a pumping session), and then use the size selector of this listing to choose the size that is 1-2 mm larger than your nipple diameter, which is measured AFTER a pumping session. Maymom is specialized in developing breast pump parts that can work with many name brand pumps on the market. The value of Maymom is to care for each mom! Maymom is an independent company. Spectra is a trademark of Uzinmedicare, Korea. Maymom is not affiliated with Uzinmedicare.