This listing is for one breastshield insert. This piece fits INSIDE the standard (24mm) breastshield to convert it to a smaller opening (21mm), which is the same size as the small breastshield. This part is NOT a breastshield - it is only an insert. This part used to be included inside the breastshields in all new Pump In Style Original Breastpumps (a pump that is now obsolete).
The 21mm size of the breastshield insert is the same size as the PersonalFit small size breastshield. It is a less expensive way to get the smaller opening without having to purchase new shields.
Medela has now discontinued production of this breastshield insert, so supplies are available only while stock lasts.