The Peg Perego Primo Viaggio Links for YPSI are designed to dock the Primo Viaggio 4-35 Nido car seat directly onto the YPSI chassis. Once installed, the links do not need to be removed, whether the car seat is used on its base or with your vehicle's seat belts. These links are compatible with the Primo Viaggio 4-35 Nido and the YPSI stroller. A second set is needed if using the YPSI stroller in double mode with two Primo Viaggio 4-35 Nido car seats.