The Allen 20mm Dynamic Block Back To Back is designed with opposing sheaves to efficiently manage two control lines at a 180 degree angle. It features a sturdy stainless steel side strap and nylon mouldings that ensure the block stays intact under high opposing loads. Its strength allows for a slight V shape, making it useful in applications like triangle backstay systems found on sports boats. One popular use is in a dual pole and spinnaker launcher system, as seen on a 49er. The block's patented bearing system, utilizing twin rows of stainless steel ball bearings on a stainless steel rail, provides unmatched load-carrying ability and durability compared to plastic rail bearing systems. Its becket design, with a large central rivet, reduces weight and provides a rope-friendly attachment point for cascading systems.