Turnbuckle
Turnbuckles are fence fittings that are used to tension wire and are often used on gates, to tension bracing wires, and similar.
Turnbuckles consist of a metal body with threaded holes at each end. A hook or eye bolt (or sometimes one of each) is screwed into this threaded opening, and these can be loosened or tightened as necessary to tension the wire.
Turnbuckles come in a variety of sizes and with different types and lengths of bolts. They can be galvanized steel, brass or stainless steel, and they can be used for various tensioning needs when building fences and gates.
Generally, the larger the turnbuckle you are using, the more tension it can handle, so choose your turnbuckles based on how much force will be exerted on them.