This product is made by Maymom to fit spectra's S1/S2 pumps in addition to spectra 9 and spectra M pumps. It is known that using the correct size of flange (breastshield) in breast pumping is the most important factor in achieving comfort and efficiency. Every mother was created differently. One or two sizes can't fit all. This 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). 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 measured AFTER a pumping session. There are improvements of this flange over the original spectra design – 1. In addition to spectra's duckbills, you can use Maymom's valve/membrane system with this Maymom flange. You can also try other brands of duckbills/valve/membrane on this flange. The chances are that they will fit. 2. In addition to fitting Spectra's backflow protector, you can use Maymom's long stem, as well as short stem backflow protectors. You can also use Maymom's tubing connector (sold separately) to connect this flange to many other popular breastpumps on the market. Content of the kit: Flange of wide mouth bottle connection x1 (Buy two units if you pump both sides at the same time). About Maymom 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! Disclaimer: Maymom is an independent company. Spectra is a trademark of Uzinmedicare, Korea. Maymom is not affiliated with Uzinmedicare.