What is a converter dolly, and what role does it play in coupling a second or third trailer?

A converter dolly (or converter gear) is the device that lets you add another trailer to a combination. It has one or two axles, a fifth wheel mounted on it, and a drawbar or tongue. The dolly's drawbar is coupled to the rear of the trailer in front (using a pintle hook and the dolly's eye, secured with safety chains), and the next trailer's kingpin is locked into the dolly's fifth wheel. In effect the dolly turns a semitrailer into a full trailer that can be towed behind another. Correctly coupling the dolly — locking the fifth wheel, securing the pintle hook, connecting air and electric lines, and chaining it — is essential to a safe doubles or triples connection.