The top tether is a tether strap with tensioning indicator for use with duo plus installing with isofix attaching to anchorage points in the car this tether strap limits the car seats rotational movement whilst travelling in the car features tether strap with tensioning indicator for use with duo plus installing with isofix attaching to anchorage points in the car this tether strap limits the car seats rotational movement and gives the seat a lot more stability compatible