SMC's Large Locking "D" carabiners are state-of-the-art. With thousands of these carabiners in service, they have proven themselves reliable. The Large "D" fits over stretcher rails. The carabiner gates and screw sleeves are constructed of stainless steel for higher strength. The nuts lock on the gate to prevent jamming. These carabiners have become the choice of many mountain rescue teams and fire department squads. SMC carabiners - "Quality Gear for Life"