A shackle or a Link are primary locking fasteners, connecting links in all manner of rigging systems, from boats and ships to industrial crane rigging, as they allow different rigging subsets to be connected or disconnected quickly.
A shackle is a U-shaped piece of metal secured with a clevis pin or bolt across the opening, or a hinged metal loop secured with a quick-release locking pin mechanism.
A Turnbuckle is a special locking fastener that holds under tension and will not accidentally open.
Turnbuckles have been used in aircraft construction, especially during the early years of aviation. Historically, biplanes might use turnbuckles to adjust the tension on structural wires bracing their wings. Turnbuckles are also widely used on flexible cables in flight control systems.
All turnbuckles are assembled with a spring mechanism which holds it under safe tension until its manually operated to release the lock. These are made from Stainless Steel AISI-316 Marine grade which is ideal for salt water and chlorine environments.