Top Tether set is compatible with Britax Duo Isofix baby car seat. Reduces installation problems and optimises child’s safety during car travel.ISOFIX is a method of fixing the child seat directly to the car without using the seat belt. It reduces installation problems and optimises protection by means of a rigid connection between child seat and car.So far, Duo Plus Isofix has been approved for over 190 specific vehicle models and any vehicle with a Top Tether.