Wire connectors are available in a variety of sizes and shapes. While their exterior covering is typically made from insulating plastic, their means of connection is a tapered coiled metal spring or interior grooves that thread onto the wires and holds them securely. 

When a connector is twisted onto the stripped and twisted-together ends of wires, the wires are drawn into the connector's metal spring and squeezed together inside it. Electrical continuity is maintained by both the direct twisted wire-to-wire contact and by contact with the metal spring or insert.