The improved mattress innerspring unit is provided which includes a plurality of coil springs arranged in a plurality of substantially parallel rows, a plurality of cross helicals extending transversely of the rows of coil springs, lacing together adjacent springs along their terminal convolutions in both the upper and lower surfaces of the unit, at least one border wire extending about the perimeter of the unit and at least one cross helical cap mounted on the end revolutions of a cross helical which cap includes a means for restraining movement of the cross helical outward of the unit. The restraining means of the cross helical cap is positioned inward of the border wire and provides a positive abutment for the end revolution of the cross helical, preventing movement beyond the border wire. A cross helical cap is also provided which is adapted to be mounted on the end of a cross helical and restrain movement of the cross helical beyond a border wire of an innerspring unit.